My Boiler

[xud]

My combi boiler is a Vokera 20-80 RS Flowmatic.

I have a number of issues with it.
1) Sometimes, in warmer weather when I switch heating on the pump starts up but the gas will not fire up (pilot light is on)

2) When the heating is working normally and I turn on the hot water tap it makes a big clunk every time.

Questons:- What part of the boiler is it that tells the gas valve to fire up?

What on earth is causing the clunk?

Is it possible to get service/wiring diagram for this model bearing in mind it's no spring chicken.

Thanks in advance