Well it's 100% the relay.
Pulled it out last night and tested it with a multimeter and with power across it there was no contact to the switched pins. New one arrived at around 1100 today, however despite checking part number and confirming model of car I'd been sent the wrong one
As you can see, the unit is somewhat smaller than the stock one (not necessarily an issue) but also has a pair of pins missing. Not ideal, but purely for the reason that I had the multimeter out I figured I'd fidget out of boredom and see if they were otherwise the same so I checked what seemed to be connected where in the new one then picked up the old, dead one to pull apart.
For whatever reason and I'm not sure why I tested the old relay again, obviously expecting nothing given it had been dead on testing last night. It wasn't.
Very odd, but it seems the relay wasn't actually fried. I assume that with the cold wet weather it'd just become a little damp and had messed things up a little inside. Stuck the heat gun on it (gently!) for 5 minutes and it's now back in the car and functioning perfectly once again. Typical!
Still, I now have a random spare relay for future use...