I need some advice on a Potterton Boiler.

It's an old ground standing unit and has worked fine since its
installation 26 years ago but is now not firing up. The power supply to
the control unit seems ok and there are no loose wires, the pilot light
is ok and I've had a look for a fuse but can't see one. I'm
therefore guessing it can only be one of two components within the
boiler itself.

The first is the thermostat but this looks ok to me. It's only a
simple device and again there are no loose wires.

The second is the main unit (not sure of it's proper name) that
controls the gas flow. This is a sealed unit that again looks ok to me
with no obvious problems.

I'm not an expert on boilers but am a reasonably competent DIYer but
I don't know where to go from here. Any advice would be much
appreciated.

Thanks.