Update:
I bought a new piston assembly from Amazon for around £3-something. Cheaper than a new waste of any type.
Took out the waste, complete with old piston.
Got a set of grips each on the waste and the piston and managed to get them to turn. They were really solidly locked - no way could I...
Sadly, if I could remove one I'd consider the problem solved. :-)
I'm coming to the view that I have to at least start by removing the wastes so I can get at them, whether I replace the piston unit or the whole waste including piston. I'm now off to do some pricing.
My house is completely fitted with click-clack wastes and they've started to fail in various ways. In theory you can unscrew the piston section for maintenance but mine are solidly stuck to the waste itself and when I try to turn them, the whole waste starts to rotate as well. I can't use too...