Okay first off. The only Mary Morston I have to go off is the RDJ!Holmes movie. And Wikipedia. And I have no idea what Brits call Kindergarten.

Enjoy :)

Miss Mary Morston was a smart woman. Very smart, she liked to think. Also compassionate, caring and understanding. She did family law after all. You had to be. This...situation however, was getting a little bit out of hand.
She had been dating Dr. John Watson for 3 months. She quite liked him. He was, well, different. An army Doctor (now GP) running around helping his friend solve crimes at all hours of the day or night yet still managing to find time for her. Time and flowers. And dates where the food is free (courtesy of being friends with well, his friend, who apparently has gotten a lot of people off prison sentences. Rightly so to.)
No, John Watson was a perfect mix of exciting, homely, caring and romantic. He was quite alright in Miss Morston's eyes.

It was his friend that was the problem. Sherlock bloody Holmes.

First date: interrupted with such style that she didn't really mind. At first. John profusely apologised later, took her out on another date. All day. No interruptions. At all. Lovely.

Third date: called off to go stitch Sherlock up.
Forth date: Same
Fifth date: serial killer (actually there really was a serial killer and they did catch him so she let it slide)
sixth date: no reason. Just Sherlock came billowing in and spirited John away.

Subsequent dates followed this pattern.

So Miss Morston had reached a conclusion. One that, if it hadn't been so frustrating and rather impressive well, cock-blocking (to put it one way) on Sherlock's part it would have been very endearing. Borderline cute. She'll have to tell him.
No, Sherlock was essentially a tad bit (quite possibly very very much) upset that he had to share John. Sharing was obviously something he never learned in kindergarten.
Did he even go to Kindergarten?
Anyway, she understood that dating John came with Sherlock. It was a package deal. John wouldn't leave Sherlock and Sherlock would, well, let's not think about what he would do if she took John away from him forever.

Yep. Time to work out custody arrangements for one John H. Watson.

New Scotland Yard. Her dynamic duo's favourite hang-out. According to the man on the phone (apparently Sherlock's brother) they were there. Terrorising Detective Inspector Lestrade.
She was expected (who was Sherlock's brother and how does he do that?) and taken up to see them right away.

Sherlock was looking very imperious and the man she assumed was Lestrade looked like he dealt with him every day. Probably did. And John. Ah John, haven't seen him in awhile, looked like he was about to either shoot himself or throw himself out a window if Sherlock didn't shut-up.
He probably wasn't going to. Shut-up that is.

"Sherlock!" That got his attention. You could see the mild confusion in his face. Who was that lady who knew him? Oh right, Johns girlfriend, boring.
So it was John who answered. "Mary, what are you doing here? How've you been. Sorry, I haven't seen you in a while but," he gestured over to Sherlock who was making a show of pointedly ignoring them. But definitely listening to every word they were saying. He was like a child when their parent meets a new partner and it was absolutely adorable. (She was probably extremely odd to think that. Probably the only person to ever think that. Still, the 'I'm going to hate you and stop you from getting close because they were mine first just because' reaction was definitely endearing.)

She held up a hand to stop John's stream of apologies. No if she got her way he could apologise by taking her to lunch.
"Sherlock I need to talk to you." There it was. The slow turn around and the look that said "shut up and go away or I will make you run off in tears" She'd seen that look when she was trying to get the kids to tell her what was going on. Seen all of the insults and tricks. None of them had ever worked. At this point.

Step 1 for dealing with Sherlock. Don't let him interrupt you.
Step 2, don't be polite or timid.
Step 3, get to your point straight away.

He opened his mouth to start a tirade. She held up her hand to shut him up. It worked less than a second. That was all she needed.

"I get him Wednesdays, Fridays and Saturdays."

Now he was confused. "What?"
"John. I get him Wednesdays, Fridays and Saturdays. I don't have to work those days so we can go out. You get him every other day." he started to protest. Magic hand shut him up again. "I'm perfectly aware the criminal kind don't operate to a schedule so, you can have him for as long as you need on a case. I'll even let you interrupt our dates if it's urgent. And by case and urgent I mean a serious one that you actually need him on. No interruption because you can't be bothered to text" Deep breath. "However, for the time spent with you when he's supposed to be with me, I get compensation. I mean a Monday, Tuesday, Thursday and Sunday with me, instead of you."

Sherlock looked stunned.

"Is that arrangement satisfactory?"
"Today is Wednesday..." he said rather slowly.
"I'm perfectly aware of that."
A moment of perfectly awkward silence for everyone else in the office while he computed this fact.
I tapped my foot impatiently "Do you need him on this case?" magic hand flourishing towards the piles of paperwork.
More awkward silence. John looked a tiny bit embarrassed (sorry, extremely) and
Lestrade looked caught between amused and well, trying to become one with the wall.
Then, with a shrug and a tiny smile (which will be denied later) "He's all your Miss Morstan," and whirls around, mind already back on the case.

Mission accomplished. now for lunch.