[DavS1]

Sorry in a rush but it looks like the slider for the caliper may be seized or just not working properly due to needing a good wire brushing/copper slipped.

the rust on the disc even once caliper carrier/slider is sorted may not go away so it'll either need skimmed on a milling machine or if you're not all that fussed a flap disc on a grinder will take the worst of the rust off and the pad may recover and eventually take off the rest (good chance it won't though) or alternatively renew the discs, but the pad may now be worn un-evenly so may need discs and pads.

HTH and i've maybe repeated what someone else has said, my bad if so!