socket live eath reversed

There is no 30mA RCD protection on the circuit else it would have tripped, and you either have a rewireable fuse or a miniature circuit breaker protecting the circuit. The live conductor has been terminated in the earth terminal on the socket and the earth wire has been terminated in the live terminal on the socket. It will have been like that since whoever installed it left it. Current can still flow through the earth wire, or cpc, which is at the same potential as neutral so it will still "work" but is very dangerous. All you need to do is take the plate off and wire the terminals correctly. As said, an RCD on the circuit would not have allowed this as it would know that current was going to earth.

I have had this before and it was a dodgy switch on the socket causing this. With the tester plugged in try and push the switch. Not push it off but push the whole switch slightly. May need a new plate.