Yesterday got a call saying the oven had stopped working, same symptoms as before. He took the back off and checked the wire, still in place. Mentioned that earlier in the day he'd got a "tingle" off the oven. The oven has it's own mcb, but no rcd protection. Told him to isolate it and wait for dad!
Called in today, isolator switch on the wall was off, so before going further I turned it on and used a multimeter on ac volts range to check between the oven casing and the adjacent sink tap (plumbed in copper, bonded to earth). 230 volts!! (he went and put some shoes on at that point!). He said he'd loosened the isolator from the wall while doing some grouting, so flicked off the mcb and unscrewed the isolator switch. Earth conductor to oven floating loose. Put it all back, switched everything on, oven clock/timer now working and no voltage on the casing. Oven now works ok, but I'm still wondering why a disconnected earth would result in 230v on the casing. In retrospect, maybe I should have put on some sort of "load" when doing the voltage check, I suspect that the voltage is coming from the "neutral side" of a highish impedance load, maybe the electronic clock/timer, but this still seems wrong to me.