Chris Wilson wrote:
I seem to remember being taught in school that metric countersunk Allen
srews had a 90 degree countersink angle, and Unified had an 82 degree
angle. Is that right, and does it matter if I just use a 90 degree
countersinking bit for both, as here in the UK no one seems to stock 82
degree bits for ferrous material? Thanks.

J&L have them
http://www.mscjlindustrial.co.uk/CES...D/product.html
and maybe others if you search.

Chronos have a set, http://www.chronos.ltd.uk item XC310 .

Didn't take much to find them after going to their sites and searching
for 82 degree.