well I got round to looking at this the other day and it seems that a dodgy relay was causing the problem. I dropped a replacement in and the fan worked straight away.
However, there is some heat damage to the vertical pin on the relay I pulled out; the fan occasionally cuts out and the controls don't operate (just like before, but it only happens for a few seconds before working again). On closer inspection there looks to be a bit of melting to the fuse box at the vertical pin part of the relay socket.
Do you reckon that damage to the fuse box is causing the cutting out now, or could there be an underlying problem which damaged the original relay?
Sodding vehicle electrics lol