No, not fixed it Tom. It wasn't intermittent for long, it hasn't worked for many weeks now. It does seem like a plague and I read the posts on the other threads.
I prised out the switch on the underside for closing the tailgate. Like the dashboard switch, this should always work irrespective of the engine being powered or not. This switch just has two wires (flimsy, not cables) going to it. My multimeter showed it to be dead. The handbook shows a 30amp fuse for the PBD, but there's just a long, green block there instead of the individual fuses depicted.
I'll continue watching here in case a solution is forthcoming. Otherwise, I'll take it to a local auto-electrician who my neighbour recommends. It's not too much of an inconvenience at the moment, but it will become increasingly important as our freedom slowly returns.