Live and Neutral reversed. However the cooker circuit was an immersion heater circuit originally 2.5mm T+E. The board was fine, but the supply to the cooker was in red and black cable from the board, however in the cooker switch it was brown and blue. So a connection of sorts was made when new kitchen was installed and tiled over. The oven and hob are both gas so the cooker switch is only operating the igniters.
As an aside and I get what people have said about the socket tester, why does it light up as if the circuit is correct when an earth prong of a plug is inserted into one side of a double socket?
Tester in one socket shows a live neutral reverse and then an earth only prong is inserted into the other socket of the same two gang plate and then tester shows no fault.