So I’ve had a little investigation into my antenna issues, I brought the antenna down on the winch took a look. It looks like the tape over the crimp connectors has reacted with the Sikaflex in some way so water may well have got in there. I stripped off the Sikaflex (which I wont use again because removing it is a sticky nightmare) and put Araldite over the cracks. After winching the antenna back up I checked the SWR trace again and it had got even worse, which was similar to my experience last year.
Looking at the balun there are a fair few cracks which I’ve temporarily protected with Araldite, it’s also not potted and uses a SO-239 so I’ve chosen to replace it with a potted balun which uses an N-type connector which are what I use elsewhere in my shack and what I’ve got on the end of my coax, this allows me to remove the N-type to PL-259 adapter.