Many thanks for all the comments.

Today the RCD blew mid-morning, without me even having been down to the workshop. By a process of elimination, I have found that an external 13A socket we never use had suffered from water damage.
I think that the water has entered the MICC which provides the supply to the workshop, and now the trip will not stay energised, even with the workshop consumer unit disconnected.

Looks like I need to either get the MICC repaired or have the whole lot replaced with SWA :-(

It may be that this water seepage has been going on for a while, so may have contributed to the earlier RCD trips.

Adrian.