Mrs ronmidg switched on hall/landing light this pm and upstairs lights fuse blew straight away (nothing has been disturbed, or anyone gone in loft for months). Replaced fuse. Light came on, fuse blew after 2 or 3 seconds. Turned light off and removed bulbs from this fitting, fuse blew as I re-inserted. Turned every light on upstairs circuit off and replaced fuse, blew about 15 minutes later. Took every upstairs circuit switch fitting off and checked, no signs of electrical burn anywhere, no frayed or trapped wires, did find one loose wire in switch for one bedroom (not the switch the mrs used) and insect case inside, but no burn marks visible. Tightened this and replaced fuse, now been good for about an hour, not switched anything on this circuit on yet. This is 5 amp fuse wire, maximum load if everything on about 250watts, load when first blew about 90 watts. Concern is there is a shorting connection somewhere in the wiring that is variable. Question is under circumstances above what is most likely explanation? mice? not heard any and do have poison down. (Can get by tonight with free standing lamps plugged into ring main. Will probably turn everything off and pull all the other fuses during daylight tomorrow when the house is empty and watch the meter.