Thanks mate, I think that was the issue

I struggled to remove it earlier as it was belting it down with the wind and rain, but I cleared what looked to be a little bit of metal gunk with a knife and gave it a good wiggle and hey presto the fans are no longer running with the engine off. It seemed much quieter on a quick spin and I have checked it once since after 20 mins and still seems fine.

I am going to order a new radiator switch anyway to be on the safe side, but really appreciate the help from you and from everyone else (I was about to order a temperature sensor switch today as the next step).