Ideal Isar H35 stuck on 'd'?

Yesterday evening our electricity RCD tripped. We narrowed it to our boiler, which after a reset did not cause the problem again and worked fine for several hours.

Today it got stuck on 'd' (hot water), even though no water is flowing. I'm sure of this because I turned our mains stop-cock off.

I have briefly checked the electrical connections inside and they all look OK. Any pointers as to what this may be much appreciated.