In message , John Rumm
writes
chris French wrote:

I'm thinking that maybe my RCD isn't doing it's job properly? (We
have a TT earth system with just one main incoming RCD - about as old
looking as the CU) The test button works ok though.

Are you sure it is a RCD and not an older "voltage operated" ELCB?

No I'm not.

There is a picture of a common one he
http://wiki.diyfaq.org.uk/index.php?title=RCD#ELCB

In fact it looks exactly like that.....
--
Chris French