Delia Busby is a jewel. Delia Busby is a pearl without price. Delia Busby, for whom Patsy would sell all she owns, and, she now realises, risk all things, is lying in her arms, very early on this August morning.

Trixie is away for the weekend, attending the wedding of a friend from training. Patsy doesn't envy the bride, whose every decision will be scrutinised by Trixie's gimlet eye. But she appreciates the fact that – this morning at least – there will be no need to pad back to their shared room in the small hours, with the prepared excuse, should her roommate wake at the wrong moment, that she was just visiting the bathroom. She can stay in Delia's bed till dawn, luxuriate in it, and in Delia's presence.

Delia is playing with her hair, coiling it round her finger and watching it spring back when she releases each red curl from her tight captivity. But she suddenly stops, looks nervous rather than impish.

"Pats – I wanted to ask you something. About - after the accident. You know I said everything came back – all of it. Well, the bits about you first, they came rushing back to me."

She pauses, unsure for a minute of how to continue.

"Delia?"

"Only there's one thing I can't remember. I know it happened, because I have the photograph of the two of us. I thought my mind was just being stubborn, that maybe once I was back here – back with you – the memory would come to me. But it hasn't."

"Sweetheart, what is it – what photograph?" She's worried: mainly, and first, and always, for Delia – for the anguished look on her face, the worry about her memory that still sneaks up on Patsy. The same terror grips her now as when she sees Delia put her hand to her head, the fear that the slightest headache could presage something worse, something more ominous. But she's also – selfishly, she supposes – worried for herself. What is it that Delia's forgotten? Their first meeting? The day when Patsy finally, after so long, worked up the courage to say something? None of those moments in their shared history is anything less than absolutely important to her.

"You're not cross, are you?"

Patsy is disgusted by her own momentary failure to respond. "How could I be cross with you? But what is it that you don't remember?" She tries to break the worry spreading across Delia's face, "though that's a difficult question to answer, I suppose".

Delia sits up in the bed and turns to the table beside it, and opens a book – it's Sylvia Plath's The Colossus, Patsy recognises from the green and white of the cover – and pulls out a photograph. It looks otherwise innocuous, as if Delia's simply been using it as a convenient bookmark, not pouring over it like some missing piece of her own history. As she hands the photograph over to Patsy, Delia thinks of the first line of that poem: "I shall never get you put together entirely/pieced, glued and properly jointed". It makes her shiver.

The photograph is the two of them, standing on a pier in the light of the late morning. Patsy is distracted briefly by the thought of how really beautiful Delia looked on that day, in the blue and white of that dress. Of how Delia's presence made the brackish waters of the Thames Estuary more sparkling and glamorous even than Monte Carlo or Juan-Les-Pins. The thrill of standing next to her while that photograph was taken is still fresh, still wonderful, even now. She exhales.

"Southend", she breathes. She's sitting up now, with Delia's head resting on her shoulder.

"Will you tell me about it? I do so want to remember it."

She strokes Delia's hair.

"Everything, Pats – all the little details. I want all of it. I want to know how we got there, and where we sat on the train, and what flavour of ice-cream we bought, and what we talked about, and how cold the sea was when we went for a paddle."

"You remember we bought ice-creams and paddled in the sea?"

"No – but I know that I always insist on getting ice-cream and dipping my toes in the water when I'm at the seaside. It's a point of principle." Delia wiggles her toes, emphatically.

Not for the first time, Patsy marvels at how much herself Delia always is.

"Southend. We went to Southend. We arranged to meet at the train station. You'd been on a late shift the night before, as last minute cover. I suggested changing the day, but you insisted you'd be fine. And then you made it to the station with only seconds to spare, and bundled me onto the train."

"Sounds like me", Delia interjects, smiling.

"It was very like you. Maddeningly like you! Needless to say, I wasn't best pleased with you nearly ruining our day out before it had even begun, and refused to speak to you for the first ten minutes of the journey."

"Sounds like you." Delia smirks, and adjusts her position next to Patsy.

"But I forgave you. Because as we were going past – well, I don't know – somewhere outside of London – you took off the necklace from around your neck, and slipped the ring onto your finger, and smiled at me."

"A winning smile."

"Of course. Nothing less. Anyway, when we got off the train – it was a weekday in September and the weather was gorgeous. You would have mistaken it for summer. And as we walked around, you started saying how much it reminded you of The Mumbles, and the Welsh coast, and talking about your holidays as a child. And I distinctly remember you telling me that the problem about the English seaside is that it has too many seagulls, and not enough castles." Patsy snorted. "Honestly – as if you didn't have seagulls in Wales. But you insisted to me that the Welsh seagulls were much better mannered."

"Well, that's true. They are much better mannered. I'll have to show you, one day."

"I remain to be convinced."

"Anyway – after that – we had our paddle and our ice-cream?"

"We did. Well, we walked along the pier first, and had that photo taken."

"You look quite lovely, you know."

"Delia Busby - you shameless flatterer."

"You do. I suppose that's what upsets me about not being able to remember it: why can't I remember such a nice day – you looking so lovely, and having you on my arm, and all to myself?"

Patsy swallows. There isn't really an answer to that – medical or non-medical. So she continues.

"So we walked along the pier. We tried some games – we were both hopeless. I think – yes, definitely – there was a coconut shy. You insisted it was rigged, said you'd hit the coconuts and the balls just bounced right off. And then we each had an ice-cream. Eaten, I must admit, in a most unladylike fashion.

And we took that paddle. Gosh, the water was cold! And both of us got sand all over our stockings when we had to put them back on.

And then, again, we had to rush for the train – because you had insisted on exploring underneath of the pier while the tide was out, and looking for rock pools. But I didn't mind too much, because whilst we were walking around – clambering over the rocks and the seaweed, carrying our shoes because it was so slippery – you gave me a very brazen kiss."

"Like this?"

Patsy's narrative halts for a moment.

"Exactly like that. I almost fell into a pool of seawater."

That information seems to please Delia.

"So we rushed for the train, and just, just made it back. We must both have looked rather the worse for wear – so it's a good thing we had that photograph taken at the beginning of the day. And both of us agreed that was a good thing we'd only just made the train, because we hadn't wasted a minute.

And then - it was quite late in the afternoon, and the carriage was almost empty, probably because it was midweek, and the start of the off-season. And at one of the early stops, a drunk got on. Probably harmless, but he sat down opposite us and started leering, and making all sorts of lewd comments."

Patsy's nose wrinkles at the memory before continuing.

"Then I decided we should move, but you just stood up and fixed him with the kind of glare that you used to reserve for the very worst patients on Male Surgical. You called him a nasty, grubby little man, and thrust your hand out in front of his face, and showed him your ring, and told him you were spoken for and to keep his opinions to himself. And then you pulled me along and into the next carriage. And we laughed all the way home."

"And when we sat down in the next carriage you stroked my hand?"

"You remember that?"

"No – but I know you. You're predictable. In the best possible way." There Delia pauses. "You know, I wish I remembered it. There's not one moment with you I don't want to remember. But I feel better knowing that I can imagine exactly what we did on our day out, even if I can't remember it, before you even tell me what happened."

"Like joining the dots. The very predictable dots."

"Exactly." Delia kissed her exposed shoulder.

It's more or less morning, and Patsy can hear the all sounds that come before the sounds of the house stirring. She thinks of that moment in Casablanca – a film which they'd only recently seen, all of them together. It had been playing at the civic centre, and they'd gone at Sister Monica Joan's (and, she suspects, also Sister Winifred's) insistence, the money from the screening being collected for some good cause. Towards the end, Rick turns to Ilsa and promises her that they'll always have Paris. At that moment, Delia had, unassumingly, squeezed her hand. And at the time – caught up in the rush of the film, and the long looks of Ingrid Bergman – maybe the idea had seemed romantic to Patsy. But now she thought of it, it was only sad really. The two of them in that film had settled for a memory. She and Delia wouldn't always have Southend (the very phrase was ridiculous, derisory, almost made her laugh), but they didn't need it. They could go back, any time they wanted, if they wanted. And she could – before they'd even bought the train tickets – imagine how good the day would be, with Delia next to her, with Delia's talk, and observations, and squeals of delight. But the lure wasn't Southend, or its pier, or paddling in the sea, or the hope of recreating that instant last year when the Thames Estuary felt like the French Riviera. She didn't need the one particular memory to feel complete, and nor, more importantly, did Delia.

She would settle for this: Delia Busby, in her arms, on an August morning. A pearl without price.