Before Christmas three lights failed in the kitchen, not all at once, but failed nevertheless. We tried to replace them, but they were the sort that fit snugly into the kitchen unit over the work area and to us, they looked like integral units. In other words did the whole units need replacing? So we asked an electrician and he came yesterday. 'You'll need a new transformer.' Right. 'I'll have to go and get you one.' Okay. Two hours later, transformer replaced, the lights still didn't work. Out came junction boxes, off came the wall switch and in went various screwdrivers, testing for circuits and ringing bells. Nothing wrong with the electrical system. 'Did they all go off at once?' No. He dismantled the what we thought were integral units and took out the bulbs. 'The elements look alright.' He disappeared up to his van and came back with a replacement. It worked. 'Must be a fault in the bit of the bulb that goes into the socket.' Oh, that's the problem is it? It's only a small thing with prongs rather than a bayonet or a screw and it does look fragile.
Three hours or so and we have the answer. Go and buy more bulbs, stupid.