I have posted this on the CCC but some might miss it so heres the solution to rusted on brake disks that just will not budge even if you whack them from the rear with a 4lb lump hammer for about an hour.
The back of the disc was force fed during this time with WD40 and some Duck oil.
I had one of those silly Idea moments
I thought how about using an SDS hammer drill in hammer mode only to put some vibration through the disk hopefully releasing it from the hub it was rusted to. Only do this if you have a new brake disc to put back in its place, I certainly wouldn't advise anyone to do this to a disc and re-use the old disk once it is damaged.
At this point it may well be worth noting that the whole set of front discs and front brake pads for a
1.9D C15 were only £25.00 from a auto factors called CT AutoParts
I put a wide chisel in the SDS hammer drill locked it to hammer only and got my Father to rotate the disk as I applied the heavy SDS chisel to the Front of the disc hub we then concentrated on the centre of the disk and
cut out a slot with the SDS hammer action and then put in some more WD40 through the cut out slot and then
hammered the back of the disc once more with the 4lb lump hammer and finally the disc
came away from the hub.
The centre of a brake disc is thinner because it is tapered on the inside of the disc making it easier to
cut with the SDS hammer action. we won our battle finally and I thought if it works why not pass it on I am sure that someone can use this little problem solver, we did consider heating the disc up but worried about damaging the bearings or cv gaiters.
Piccy of the chizeled out Old Brake Disc Below.

Regards Nigel.