Old skool : Dot 4 brake fluid is the best penetrating fluid I’ve ever used. Brush a bit on and leave for a while. Penetrates far better and doesn’t evaporate for a long time either. V8 or else ...

Thanks all. I shall persevere.

Ended up taking it to a local garage, who phoned me 20mins later to say all 4 bolts freed off, cost £20.
Sometimes worth it to admit defeat

Turned out the old disc had corroded to the point where the connecting bridges joining the two halves of the disc together had literally crumbled over a third of the disc. So pressing the brakes actually squeezed the two haves together. Explains why the front of the car felt ready to fall off!