Any bright ideas? One of the pistons won’t quite push all the way back in, and it’s stopping me from fitting new pads.

Do these tend to get muck in the bores, or is it a case of cleaning the outside of the piston?

It may be that the brake has a bit too much oil in it. This can happen when the hoses are cut and the pistons not put back into the caliper housing.
Try opening the reservoir cap and then gently and slowly pushing the piston back. (You will need something to either suck out the excess oil from the reservoir at the lever or a cloth to wipe up the overspill. After the piston is in place, fill the reservoir if you need to till it’s level and replace the membrane and cap so that there is a slight overspill. Fit the new pads in, replace the wheel on the bike and pump the lever a few times to get a bite point. 😉