Thank you for your quick reply!
As I said he seems to be healthy other than the strange fins so perhaps it is just a deformity that he’s adapted to living with, he doesn’t seem to be in discomfort or struggling to swim at all and is still eating and alert just like all the other guppies. He’s even gone as far as to be nipping his rank mate in the hospital tank (I’ve separated them now that I’ve noticed him nipping).
As for GBD I can’t see why he would have this, none of the other fish in the tank show any symptoms like this at all so I think a deformity sounds like the most reasonable explanation. If this is the case I’ll stop treating him and put him back with the other fish