There’s power to both bits of the relay with the ign on so I checked it directly on the battery and it doesn’t switch, at the moment the only bit that seems at fault is the relay itself. Someone must have had some knowledge to bodge it as it took me 15mins of studying the wiring diagram to work out what was going on.
It may also explain why I thought the lights flickered a couple of times when the heater flaps moved, I just thought I was seeing things.
I think you’re right about it being a lucky save.