Remove the rear caliper and disc,
unscrew the sensor and clean off any rust making sure the area the sensor sits on is free of rust clean and flat.
Then clean up the magnetic ring with a cloth and brake cleaner or something similiar
Then just refit everything
think some guys do it without removing the brakes but i think i prefer doing it this way
Just a thought i know garage fitted rear discs and like you i would draw the conclusion that the abs fault was the rears but how sure are you its not the fronts
Also i think when i fitted discs on my 307 there was with and without abs rings available hope they fitted correct ones should that be rhe case. You might have checked on ecp web page with your registration number just to see if with and without are available for your car
Hope you get it fixed worrying time when you need your mot :thumb:
I would assume its the rear only because there were no light previous to new rear disc,s fitted,yes my car needs the ABS ring i did ask the garage to make sure they have fitted the correct ones to which they said yes.