Anyone had to do this? My rear piston decided to collapse when changing brakes and discs...the odb reader wasn't retracting the piston and a simple feel of the piston led to it falling out of its seat...

Is it a simple job? Happy to do it or am I going to spend a fortune limping to garage down the road...?

I've done it before on other cars. At this point it's probably worth removing from the car. You can't drive it like it is anyway so make your life easier. It could be as simple as pressing back in but likely will need some sort of disassembly. Maybe look into a refurb kit (seals, etc). But then by the time you've done that you may as well just get a new caliper.

New caliper solved the issue - easier all round!