If I switch the dishwasher off and reset the RCD it just trips again.

How do you know if it is the dishwasher if it trips when it is off? Try
unplugging the dishwasher to make sure it doesn't trip any more, but does
when it is plugged in.

If it really is the dishwasher, it could be almost anything inside, although
dodgy heating elements are well represented as the cause.

Christian.