So, on taking them apart in daylight, I've discovered the problem. One of the wires from the transformer to the track mounting points is burned through. I'm not sure how or why this happened - anyone care to suggest? Could it be low quality bulbs? I've always made sure to replace like with like, but have put in a couple that have been a different bulb manufacturer at some point (same volt/wattage/R-number etc).
I'm unsure exactly how it's all cabled together up there, and not sure if I removed the light fitting from (presumably) whatever screw-block connects that to the light circuit up there whether I'll then be without the other set of lights too.
That would enable me to see if it's feasible to replace this bit of wire (with something a bit thicker, perhaps?) or is it likely to be a case that the transformer is faulty and caused the wire burnout? it's only one out of the 3 or 4 that are in there.
It looks like it might be possible to get hold of another transformer (certainly there's a manufacturer etc stamped on the existing) so I'm hopeful even if that's called for it might not be the end of this pair of lights (I really do like them, and can't afford to shell out for new ones really).
Thanks for all the help I got here, it's much appreciated.
Velvet