Thanks, Dave. I ended up swapping the circular fan element, because I’d already ordered that. I wasn’t expecting it to fix things, but my brother (who understands electronics whereas I don’t) said it could still be the fan element even though the rcd tripped when the grill was on (not the fan element), because the element isn’t ‘isolated’. Anyway, for anyone else reading this, replacing the circular fan element seems to have fixed it even though
a) The first time the rcd blew it was about 5 minutes after we had turned the oven to ‘off’
b) Another time we got the rcd to trip even when not using the circular fan element – we were only using the grill – and it blew after about 4 minutes of using the grill once things heated up, not straight away.