Alan,
If they are still the mirrors that I fitted you can (very carefully) pop the glass out and get at the head of the screw from the inside. I would recommend warming the unit to make the plastic surround more pliable.
But I also recall one of the glasses (left, I think) kept leaving the frame of its own volition so I eventually added a bit of non-standard black silicon goo to persuade it to stay in place. That one may therefore be a bit more difficult to shift – I believe I may have a spare glass – I shall check if you are interested (or possibly desperate).
B……….
Anonymous
Bernie
David Faulkner