Working on a house today where cables in the loft look like they were from 1960's. 2.5/1mm with green rather than green/yellow earth sleeve. The cable giving me most concern was 2.5mm, buried in insulation and next to hot pipes.

The outer sheath felt very soft and malleable and had an oily coating, no green gue. Insulation resistance about 90 M Ohms.  My interpretation is that the outer sheath at least is breaking down, inner insulation felt ok. Would you agree the cable is beginning to fail, or is that how it was made? From what I have found on line the sheath could be Polythene rather than PVC, which I guess could be softer, cables were probably exposed in the past, but discolouration also under the clips, white plastic clips, could be newer than the cable.

At the same time I found a circular junction box that had a clear liquid formed on it's inside, not green gue or water and not from what I could see coming from the cable, this caused an insulation failure at 0.03 M Ohme. My assumption is that the plastic of the junction box was breaking down. This is a new one for me.