No HW or CH suddenly from my Vaillant T3WF H Combi Boiler?

This morning my boiler suddenly stopped supplying both Hot Water and Central Heating for no apparent reason. Everything sounds and looks OK with the bolier - pilot light is on, the boiler roars into life when switched on, it starts up when the hot taps are opened around the flat, but it just doesn't produce any heat. It's 25 years old and has worked beautifully until suddenly this morning. Anyone got any ideas?