So...electrician has been round and identified the problem. A bit of a brown trouser moment for myself and darklittlefox.
Essentially the 2 power points used to be a cooker power point. When they converted it they "forgot" to wire up the earth. Something plugged into the sockets has an earth leak. It's unlikely to be the microwave - that's something I've had before I moved in and it's never been a problem before. The dishwasher and washing machine are both new. My bet is still the 4way - it looks really cheap.
So he thinks we should in the short term, not use the sockets and use one of the other power sockets. I'm hoping this means he's checked them out and found them to be OK. Long term he's guestimating about £180 to rebuild the power sockets.