Managed to get the old bolt heads cut off with the angle grinder, and got a new mount fitted up. The left top bolt was a bit awkward to get to with the disc but with a bit of jiggery-pokery I got it chopped off.

Once I finished off the installation of the new Terrafirma shocks and HD springs, I remeasured the wheel arch height and I managed to gain about 40-45mm on each side at the back. Well worth the change.

Thanks for the help once again guys.