If it is the heater element I assume its on the bottom of the drum, is
it an easy fix?

Easy enough if you like that sort of thing. When the instructions say
"loosen the nut/bolt and withdraw the element", this should be replaced by
"swear loudly and hack at rubber grommit with a Stanley knife before
patching wounds at casualty". Apart from that, it is an easy job.

Sometimes mineral insulated heating elements get a bit damp. This can be
fixed by sticking it on a 90C wash whilst plugged into a non-RCD circuit. If
this doesn't fix it, replace the element.

However, the fact that it only happens on the hot washes could indicate a
different problem. For example, most washing machines only hot fill on 60C+
washes. If there was a live-earth fault on the hot fill valve or associated
wiring/circuit board track, then this would blow the RCD every time you did
a 60C wash.

Christian.