@kieran-hutchison Lots of ways of doing this but my preferred method is to get the caliper off the bike (but leave the hose connected for now), then gently pump the piston as far out as possible just using the brake pedal. Stuff some cardboard between the piston face and caliper body just in case the piston pops out suddenly and try and resist the temptation to grip the piston with mole grips etc. as this is a sealing surface. Keep an eye on the reservoir level as if it gets too low and air gets in, you might need to do a brake bleed just to get the piston out. And cradle the entire caliper in an old towel etc you no longer want to contain the spilt brake fluid. I've also used a foot pump before by stuffing one of those plastic airbed adaptors in the threaded hole where the hose attaches, this works well but you really do need to be careful and pump slowly as the piston will pop out suddenly, so you really need more than cardboard to stop it being damaged (and wear goggles ideally). In both cases it's better to pump the piston as far out as possible without it popping out, then edge it out with your fingers when it's almost free if you can.
It might be the case that a previous owner rebuilt the caliper with an aftermarket piston and seal kit. I bought an eBay DTR rear caliper rebuild kit once, cleaned the caliper completely including the seal grooves, put it together and bled it and there was a small dribble of brake fluid on the back of the pad on the piston side. Wiped it off and it came back several times, so I cleaned up the (old) genuine piston/seals and put them back in as I wanted to go for a ride that evening and it cured it, no issues for a couple of months whereupon I bought new genuine ones. Compared the seals, the eBay kit seals are slightly smaller in cross-section than genuine which is what causes them to leak.
