So today, having turned and tested the circuit off, I pulled down one of the downlights in the kitchen, and some of the insulation of the downlights wires was gone, leaving bare wires only a couple of mm from each other, as the pics show. After seeing this I checked the other downlights in the kitchen, and they also had damage, but only about a mm of bare wire showing at the base of the lamp holder. The other downlights downstairs in other rooms had no bare wire, just the outer fabric sheathing was slightly scuffed. I'm wondering what caused this damage; I doubt it was rodents, but my theory is that when the house was built the downlights had been pulled out of and put back into the ceiling carelessly many times, perhaps causing this? Any ideas as to how this happened?