Consumer unit tripping and hard to reset
As a follow-up, there were a couple of issues:

The RCD was broken. It was tripping correctly, but it should have been able to be reset. This has been replaced and now works as expected (and as an aside, I'll soon be replacing the whole CU).

The washing machine was defective. Once the RCD was fixed I was able to perform some tests. The heating element had failed. Disconnected it stopped it tripping the RCD. I've installed a new heating element and it now works perfectly.


Thanks for everyone's help &amp; suggestions.