Why won't my gas water heater light?
The good thing is - the nice man from Scottish Gas fixed it for me. Diagnosis took about 20 seconds - he knew before he had the cover off.

He couldn't hear the impeller fan running. Turns out the motor was over 16 years old and had just died. Because it wasn't drawing any air, the pressure sensor kept the ignition circuit off and the gas valve closed as a safety measure.

One swift replacement later (he had one in the van) and it worked straight away.