Just read your PM but thought I should answer on here (may help others in the future)
Basically I did what Keith described but hammered a 12 point star drive bit into the mangled 'allen key' head. It's so long ago that I can't recall what size bit I used but I experimented with one of the removed allen screws to get the best match.
A torx bit may work just as well but the 'splines' are a bit coarser so it could be more difficult to hammer in. ( think the hammering impact helps to slacken the seized screws)
To minimise the risk of the bit slipping I squared up the star bit and extension bar the used an axle stand as a rest to maintain alignment while applying the torque.
The replacement screws are 12 point so the problem should not occur in the future
Cheers Spike