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Not sure if this is problem or not - when opening up a socket which seems to be on ring main, outer grey wire seems to have a tarry goo on it. Stripped cable and on inside of grey cable is clean. bit concerned cable is melting but could this have been there from fitting or is melting a possibility. No fuses have tripped, socket tester says socket is OK, running off 30 amp fuse on fuse box. think the wire is twin & earth 2.5mm squared

You can sometimes get this problem with old cables made in the 60's/70's.
here is an extract from elsewhere on the net.

Its origin is the plasticiser used to provide flexibility in the PVC
polymer compound. This is generally di-octyl phthalate, which over time
or with excessive heat has reacted with the copper conductors to produce
copper phthalate (hence the green colour) suspended in the liquid
plasticiser. The material is of health concern, so should be handled
with care – gloves should be used and waste disposed of properly.
Industry guidance is generally as follows, if green gunge is seen.
Although there does not appear to be a problem with the electrical
performance or safety of the cable itself, any exuded gunge should be
removed as it can cause corrosion or affect the action of switches and
terminations, potentially resulting in tracking / overheating. It can
also cause cosmetic problems such as staining.

Thanks but cable isn't as old as 60's/70's and there is no green tar, just grey but thanks anyway.