just investigated further, and looks like yes, its a sticky piston. one works fine the other doesnt, but once its out a bit, it doesnt go back.
ive had it out a few mm, cleaned it with disc brake cleaner, then a few drops of dot fluid, but still the same. mebbes needs servicing.
and yes, it probably has got all the accumulated sh*te back in the seals 🙁 wasnt really aware that i needed to clean em before stuffing them back, but i do now. dur 🙁
as it stands, ive put an old set of pads in again, and its rideable. and ill take another pair of worn pads in case i need another set. not really the way to go about things, but itll hopefully get me through tomorrow, then ill have to sort it properly.
thanks 🙂