Replacing MB rear brake pads

[NZ XR6]

I've bought a set of rear pads (Remsa Ultra ceramic) and had a look online for tips on replacing the pads. I'm not planning to replace the discs as they are well above the minimum thickness.

But it seems that it can be something of a mission to get the calliper off, as the piston can't be pushed straight into the calliper and the pads can get stuck on the lip on the outside of the disc.

If you have replaced the rear pads, how much of an issue is this? And can anyone recommend a good tool to retract the piston? I had a Stanley tool, which was OK, but was stolen with my tools when some low life scum burgled our house a couple of years ago.