I think the correct (original) allen bolts have a tensile strength of 12.9. This is much stronger than the 'average' high tensile bolt at 8.8 and can be done up much tighter.
I have had similar problems on Suds with inboard brakes where the allen bolts go through the discs (later models). My theory is that 'spirited' driving moves the bolts slightly under hard braking and gradually loosens them off. Eventually the movement in the disc is enough to act like shears and cut the bolts. The 33 with outboard brakes doesn't seem to have the same issue.
My solution is to use new bolts, a really high quality 1/2" drive allen key and a long extension to do them up with a ratchet torque wrench from under the front wing.
Sorry, I haven't got a manual handy and can't quote the torque figure, but it is pretty tight. I wouldn't recommend re-using the bent bolts. Check the bolt holes in your front discs to see if they have become oval with wear and therefore more likely to allow movement.