Mine had 4mm allen key heads and were already rounded so i tried tapping around with a small screwdriver but that was a waste of time.
In the end i had to use my Dremel drill with a flexi shaft and a cutting disc, got the head of the bolts off and removed the hugger but it was a real pain.
Blow torch on the bolts and wire brush all the crap off then soak with WD40, blow torch and a pair of mole grips just managed to shift them but there was not a lot of stud to grip.
New bolts with 5mm hardened allen heads,copperslip and spring washers but i was happy not to have to drill them out because that looks like either a strip down of the rear end or a swinging arm out job :thumbup