how to fix lukewarm water and make hot?

I have a Sime Friendly boiler in my new house and am having problems with the hot water - i.e. it's luke warm at best. I have tried the timer setting (doesn't work) and the on setting (boiler fires up but water is tepid). The central heating always comes on too. I have tried turning the central heating temperature down (dial on front of boiler) and the hot water temperature up to max.

I have noticed that when I turn on the tap, it is cool-lukewarm, then there is a burst of very hot water, then it goes back to lukewarm again.

Any ideas on what's wrong with the boiler? And how do I get a manual - I have searched online but cannot find access to one.

Many thanks,
Liz (first time buyer)