i personaly don't think there is a problem with my boiler i think it's the shower that's S***
Stewart you seem to be sure you've diagnosed the problem yourself before asking about it, and don't want to give any information for the advice you say you want. Then start complaining about answers, some of which are lightheartedly commenting on the poor behaviour of yet another combi shower.
Not very bright, is it, when you haven't got a clue about the subject?
So shove your attitude somewhere dark, please.
What flow rate do you get from the shower when it's turned to hot?
What model shower/mixer is it?
How low a flow rate at a tap will bring the boiler on?
Do you live in a convesion flat with a shared mains supply?
Does the flow fluctuate when other people use their mains water?
How old is the combi?
Do you live in a hard water area?
Have you had anyone check the boiler?
Why didn't you get it looked into 3 years ago when the shower was installed, or if you did, what was the outcome?