As (I think) Tony pointed out earlier, the elements are cheap enough to replace so I've ordered another one.

I haven't taken the old one out to bench test it at mains voltage, but I can say that I have run two cycles with the element disconnected and neither time did the RCD trip. I have then run two cycles with the element connected and both time it tripped after a minute or two (presumably when it went to heat).

On the basis of balance of probabilties and cheapness to replace, I'm going to try a new element and will let you all know what happens.

Thanks, as always, for the clever advice.