Either a sticky piston or caliper not aligned properly.

I have a feeling it could be the piston? Any ideas on how to sort it??

Cheers, purist

Clean out areas where the pads sit. Put an old set of pads in with a screwdriver between, pump the lever gently until pistons are out as far as you dare. Squirt lots of WD40 or GT85 type stuff in. Lever pads and pistons gently back in. Repeat until free.
Do not get WD40 on any braking surface that you want to use again. Once you are satisfied that both sides are free clean out thoroughly to remove all traces of WD, repeat until sure.

I am not responsible if you push the pistons out so far that you can't get them back in or if you damage the seals. Neither of which you should do if you take care.
Have fun. 🙂

Or you have one heavy arse-cheek?

[i]KEEP the GT85 well away from the pistons, use silicone lube or brake fluid, either Dot/mineral depending what the system uses....dont use GT85.