Hi,

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.

I would be inclined to think that as well. 10KW is a lot of energy -
probably enough to meet the entire heating needs of a medium sized house
at this time of year and I have to wonder where it is going without you
noticing something at least getting hot and probably catching fire.

One place that you might be able to loose that kind of energy for a
while would be heating the ground around your earth rod.

Martin.