Thanks for all the suggestions, much appreciated!
I should rule out the Campagnolo Delta type brakes, they'll likely cost more than the bike in total so far.
Although the Shimano centre pull, aero brakes look interesting. But I'm not sure about the height of them (above the main pivot/fixing bolt). They possIbly look a bit too tall for the head tube, and would possibly extend forward from the bottom bracket??
So I'm going to try a cheap and light Weinmann 500 type calliper on the rear. An keep the Gran Compe aero calliper on the front. So I'm on the lookout for one of these with the cable guide on the right hand side. If any one has one please feel free to PM me.
Next step is to have a look (probably inside the bottom bracket?) and work out the cable run for the internal gear cable. It enters in the down tube without an outer, and exits at the end of the chainstay.
*2008 Rock Lobster Team Tig SL, MTB.
*1956 Raleigh Superbe 3 Speed
*Mid '80s Dave Hinde SL Lo-Pro/TT.
*Mid '90s Menet SPRL "team bike" Neuron 7410 DA.
And a few project bikes.