There are a couple nuts on old Raleigh bikes that no SAE or metric wrench that I have will work. Adjustable wrenches are fine, but the nut on the seat post clamp is in such a space that an adjustable just doesn't work well for me. I took a cheap 1/2" open end that I acquired somewhere and filed it till it fit the nuts. Then painted the end red for easy identification. My recollection is that it fits the seat post nut, the axle nuts, and the handle bar clamp nut. It might be just the right size for the clamp nut on the old Gary Burgess stems, but I don't recall for sure.
I find that 9mm wrenches and nut drivers work on some fittings -- must be a close match to some Whitworth size as I doubt that anybody in Nottingham knew what 9mm was.