I think I've got it now. The cable MUST reach the pinch bolt from below. Therefore the cable could run under the bottom bracket in cable housing in which case two cable stops are required: one at the bottom of the seat tube and another integral to the front mech (Shaun's Suntour/early Campag suggestion??).
OR run the bare cable beneath the bottom bracket through a braze-on/screw-on/glue-on cable guide(s) and up to the pinch bolt. I like the sound of this option because any front mech should do the trick. I wonder how Ian Steel solved it?
I hope that's it anyway... in the meantime - cheers guys , thanks for your help
'53 Viking Tour of Britain. '89 GT Karakoram. '91 GT Karakoram. '93 Raleigh Nitro. '95 Orange C16R. '98 GT Avalanche LE. 2x'00 GT Avalanche 0.0. '09 Boardman Pro hardtail.