I took the caliper off with a view to replacing the brake pads and discovered that one of the sliders in the rear brake caliper hanger is totally seized. I tried heating it with a blowtorch and then trying to turn it in a vice but to no avail.

I’ve ordered a new one today that comes with new sliders but is there any way to unseize the existing slider. I have put the old pads back for the time being (I can’t get the caliper back on with new pads).

More heat. Induction heat is the cleanest, safest way these days - pop into your local friendly garage with it and see if they will free it off for you. I had the same issue back in November and like you ended up with new hanger, new slides, new bolts, new discs ...and new pads. Happy days (or it will be once the new chassis arrives!!! Grrrrrr)

I just done mine tonight my osr was sized solid but I have oxi/acetylene to roast it no other way I'm afraid

My small blowtorch was probably not hot enough! I’ll have to wait for the new ones and a suitable weather window to turn up.