Add to that - I think I've isolated the issue to the radiator fan connector being faulty. 0v runs through it. I replaced the relay already and the fan fuse is fine according to multimeter.
I might have to buy a new connector for £9 and solder it onto the existing wire. There appears to be a clip-on for the connector, which in picrel is the black housing, but I think it'll likely be a solder job.