I thought of suggesting that, and it would certainly do no harm to do it - it's just possible that he will find a low enough resistance (as you say anything 'sub-infinity') to indicate a fault. However, I rather doubt that he would find that, since he says that the RCD trips do not occur until the element has been running for 5-10 mins (hence extremely hot). Indeed, even IR testing the element when cold might not reveal any problem.I think the first thing for the OP to do, given that we know he has a multimeter, is to make a resistance test between the element and its housing, using the multimeter on its highest resistance range. Anything much less than off the scale means it has a fault. Of course this will only be a test at 3V or so, so an infinite reading doesn't mean the element is OK, but a low reading will certainly prove it to be faulty.
Kind Regards, John
Thanks everyone for the replys.
Im going to bite the bullet and try one more element.