Turns out it's an intermittent connection somewhere around the fuse box. After dismantling the fusebox to look for poor connections, it turns out there one hundred and three separate components, not counting the nut and bolt that attach it to the battery...
So having put it all back (and it only goes back one way; if you have the wrong links in the wrong place or the wrong way around, the lid doesn't *quite* shut) and everything was happy for half an hour, it threw the same error.
So had a fettle at the multi-way connectors at the back of the box and it seems - for now - to have got better.
We'll see.
Even though the relay is now clicking and the ECU thinks it's all wonderful, the fan doesn't actually come on at slow speed (perhaps the same fault as the coupe's slow fan failure) but it doesn't seen to bother it and the high speed fan works.