Baxi solo WM 50/4 RS fails to ignite?

We've just moved and have a Baxi Solo WM50/4 RS which I believe from looking at sites is probably about 10 yr old. It has a Grundfos pump and ACL switchmaster clock attached. The boiler fired up initially but when we turned the temperature knob inside the boiler casing down, the boiler refused to reignite when temperatures require. The engineer couldn't see a problem at a quick glance and switched all knobs on and off in random sequence and boiler fired up. We then altered the clock a few weeks later when BST ended and it played up again. Again the engineer called, twiddled knobs and switches and it fired up. Turned it on to constant this morning, later turned back to 'timed' use and it refuses to work. We plan to replace it with a combi in the new year as will be more efficient for us but in the meantime, twiddling knobs and switches ourselves has not worked (even if is cheaper than calling out the engineer). Clock is working OK, pump sounds like working OK, just boiler steadfastly refusing to work. Any suggestions? All help welcomed. Any clues which bit isn't working - the engineer said it could be one of many! Thanks.