Well I had a spare copper Kindred V2 that I never really used, and took the switch apart from that.
The magnets were not corroded on this one, so I switched them with the ones in the SS Kindred that were really corroded, and now the button is not getting hot and not sparking so all seems to be working.
It must have been the magnets where they were corroded and showing the layer underneath the shiny part that was darker, possibly this was causing resistance in the switch?
Since I switched the magnets the fire button is working as it should, and I am pleased as it was my favourite mech.