What shifters?
Sora 8-speed.
Sora brake/shift levers are not designed to index a Suntour front mech. The combination not working properly isn't surprising. I would replace the front mech with a Shimano road front mech, possibly a Tiagra triple.
Well, I went and acquired a Shimano Tiagra Triple FD, together with an adapter, as the mech could only be sourced with braze-on clamp, and have just not got it to work properly at all! After lots of fiddling, fettling and swearing, and the mods mentioned below (so many variables here!), I tried the previous mech again, and it's working pretty sweetly.
I'd also suggest that the range over the chainings may cause a bit of an issue too.
You were quite right, PB. I've acquired a 46T outer chainwheel, so the triple is now 46-38-26, and that's helped. I've also applied rivets to the outer and middle chainwheels, which provide some purchase for the chain as it is shifted, as TA chainwheels are otherwise as flat and featureless as a pancake (more so actually, as pancakes aren't generally polished!).
I'm sure the set-up can be improved further, but this is a significant shift forward (no pun intended!). I'm certainly leaving any ideas about Nokon cable outers until all else has been tried. It's now time to give it all a try on the road (solo to start with).
Thanks for the help everyone.