Thanks for the advise guys! I got it eventually, I used 2 pairs of vice grips with a rubber pipe around the piston as to not scratch it, soaked it in wd40 then came out in the middle of the night when it was freezing and hit the bolt with heat and it came out easily.
The problem was the washer locks to the piston and the washer has a notch that sits in the coil top hat, and locks them together, ( I don't know if it's called a top hat but I mean the top cover that holds the spring) . Something had spun and rounded out the notch that's why the piston was allowed to spin. The other side of the car was easy and only took a few minutes, I could just hold the top hat to stop the piston spinning while undoing the nut..
I welded the washer to the top hat for now so I can remove it easily for when I get a replacement hat and washer, I installed the new strut mounts and I'm really happy with how it drives, I had removed the power steering to make room for the big turbo and I was hanging of the wheel to turn it, it's heaps easier to turn the wheel now! Almost feels like it's got power steering again!