Why is my Potterton Netaheat 16/22 cutting out?

Hi - can you help? My Netaheat 16/22 Electronic is cutting out. It was installed in 1987, serviced every 5 years-ish... (last flushed+Fernox in '96) . It starts normally on the twice-a-day timer schedule, runs fine, heats the hot water and the rads OK, then after a few minutes, it cuts out and won't come back on again in that cycle, for rads or hot water. Gradually the rads cool down and we begin to freeze! The fan and the pump are still running and I can see the electronic spark sparking through the window, but the main boiler doesn't want to light. Could it be a faulty gas valve (hard to find+expensive 907219)? Or boiler thermostat? Or flue (the wisteria tendrills grew into it last summer and had to be hauled out). Or something else? I have a mate who's a Corgi and will do the work (I know the Netaheat's the Widowmaker!), but he's not familiar with this boiler (being more used to modern top of the range German kit). Any suggestions (it's cccold in here!)? Cheers, John