Update so far.....
Universal caliper tool turned up yesterday so had the caliper off again today. Wound the piston in, this went in fine, not seized thankfully. Was a bit worried it may have been a seized caliper, but thought it wasn't because the handbrake worked fine on that wheel.
Cleaned off all the build of of brake dust and a little corrosion that was on the clip that holds one of the pads on the piston. There was no copper slip on either of the pads so put some on. Then also put some silicon grease on the slider pins. Put it all back together and so far so good. Hopefully that is, just needed a good clean and re-lubricating.
Before, I could drive literally 100m down the road and the noise was there. Did a couple of miles round the block and no noise so fingers crossed thats it.
Got a 200 mile round trip to Bristol this weekend, that will test it!