[Hauki]

Forgot to mention, there was a metal disc like cover that goes behind the brake rotor that was pretty rusted and this looked to be touching the rotor (This would explain the noise I thought was a wheel bearing recently). So I removed that.

IF it was that metal on metal contact making everything hot is it possible that it made the caliper so hot it could boil the fluid inside and this would push the piston out?

As when I stopped the car it did seriously stink of burning brakes. Which has now disappeard thankfully.

The sliding pins were moving freely and the piston seemed ok so it was a strange phenomonen and can only be attributed to heat build forcing the piston out I think.