Well, I'm guessing the enclosed is the problem.

It was the plug on the surge protected extension lead.
I've replaced it with a proper one, (ie not one pre-attached to an extension lead bought from B&Q).
I'll have to replace the socket, too, as that's been fried internally, as well.
And I'm assuming that's why the RCD was tripping, rather than the fuse of the circuit breaker for that socket circuit.
Just checking, am I right in thinking that?