Thanks guys, I have earmarked the nutted R559's as a possible solution (a good dual pivot brake by all account and not overly expensive). But I was wondering if there was anything with a bit more retro feel that folk were using. It looks like the Mafacs (Racer??) might be a possibility; I also like the look of the Weinmann long drop centre pulls. I'm picking brains to see what else is out there that will do the trick.
What reach do you need?
Some of the CLB calipers have a very long drop, I've just used some on a Rotrax rebuild which is converted to 700c, although they are not that easy to come by. You could try posting in the wanted section. The problem usually arises with frames built to allow for mudguards, a lot of these frames were already fitted with long drop calipers which are usually 47mm-57mm. Changing the wheels to 700c often doesn't leave enough reach with the old caliper.
There is a bit more choice with centre pulls like DiaCompe 750 or Weinmann 750's, Weinmanns are fairly plentiful.