Its all in the game
Its Sarah's christening and everyone is celebrating when there is a knock on the door which Blaketon answers to a policeman who looks lost. He recognises Blaketon and introduces himself as 'PC Graham Edwards'. The look on Kate's face tells us that Nick's replacement has arrived. Phil and Graham are asked to go undercover to investigate a well known gambling club. Blaketon thinks that Mr Armstrong will cheat in an upcoming race by tampering with the favourite horse 'fab four'. Armstrong notices that the police are on to him and he takes Kate and Sarah hostage but they are rescued by Graham. It convinces Kate that Graham is not such a bad guy after all.
Well the bases of the story are the same it's just the characters that are different. There is no Gill and Kate and Sarah were not taken hostage. The new character Graham is probably the same as Jo just a better character and would of worked with Kate.