My old combi boiler refused to fire up today. It's done it a couple of
times in the past and seems to sort itself out when left alone.
However I'd like to get to the bottom of it.

The pilot light is lit.
All the switches seem to work. (I can hear relays operating)
According to my multimeter the boiler control thermostat is working
and joining (shorting) the two terminals to the thermostat doesn't
have any effect.

The problem is when you click the switch you hear the relay, you hear
something else happening (low hum) but there's no big flame to heat
the water.

How can I test the gas valve/pump? Is there some kind of reset switch?

Needless to say I'm not going to risk doing plumbing work myself but I
can't to much harm checking and cleaning up spade connections etc.
Obviously, I want to have my own diagnosis before calling someone else
in.

What's my next step? Anyone help with specific help or general boiler
diagnosis advice?

TIA.