In message , Dan
Gravell writes
Peter Parry wrote:

Allen keys come in both metric and imperial sizing - is it possible
you have used the wrong sort? If so the correct type might just
still work. (To make life interesting they are not marked in any way
so you can tell which is which).
If it is the correct one sometimes flattening the end on an oilstone
to give it back a good flat surface will enable you to get a bit of
grip.

I bought the key in a hardware shop, where neither of us (me nor the
shopkeep) knew what size it actually was. It just came in a wooden box,
I bought a few and used the best fit. Looking back this is probably not
the best way, but you live and learn. I'm a DIY noobie, so be gentle on
me

I'm pretty sure even if I flattened it it would not work, the screw is
more or less circular inside now, and the key itself seems very soft as
that has also warped slightly.

You could always carefully drill it out

--
geoff