Thanks all for the advice – I think we knew the answer but were frustrated about having to take up some of a partially restored floor (which clearly isn’t the right attitude!).
We’re presently having some building works done (completely different are area often house, so not the cause of the fault), and one of them agreed to take a look. We got the floorboard up and discovered a loose earth wire in the junction box. This has been fixed and brief testing appears to suggest that the issue has been resolved, however the electrician is also coming out to take a look.
We are about to have a new RCD installed (related to the aforementioned building works), and so I wondered whether we could use this opportunity to build in any extra precautions? Especially considering we probably have a few junction boxes dotted around. I’ve read about AFDDs to help protect against arcing, but I’m not really sure how they work? Will one module added to the RCD cover all circuits, or do I need one for every circuit (i.e. in place of the traditional type)?