First (with the power off obviously) rule out a problem with loose wiring in that socket causing the socket itself to overheat and for that to overheat the Solwise. I suspect it's not that because a homeplug draws very little power but it's easy to check. Perhaps there is something else plugged into that socket as well?

A bit of googling finds several makes of homeplug have had overheating issues and it looks like Netgear recalled some of theirs. There are one or two reports of Solwise also overheating but reading between the lines it appears that the problem might be at least partly down to the electrical installation.

It appears that it might be more difficult to transmit signals on the mains in some houses/sockets and that this might be partly the reason why some overheat in some sockets and not others. My feeling is that when plugged into a "difficult" location they up their power output and that causes them to overheat.

To be safe you should probably return the unit(s) and report that it's overheating, but perhaps don't be surprised if a replacement does the same in the same house/socket.

You could also perhaps check there isn't a wiring fault using something like a socket tester but they can't reveal all faults (eg I don't think they can detect a poor earth) so perhaps also worth getting the electrics checked.

Thank you, that was exceedingly helplful.

Will certainly post on the Solwise forum.

I posted here first because the 85PEW is working fine here, but not in the other house, notwithstanding no other equipment was in the socket, wasn't covered with anything, nor was the bottom sitting on the carpet (ie penty of fresh air around it).  So I thought there must be an issue with the electrical system in the house?  Will get them to test the socket.

Thank again.....