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They are Veloce shifters, and they do indeed have two routes. I have used the lower path, and routed the cable behind the bars - which is something I don't usually do.
I have removed the bar tape, and this led to an immediate improvement - the tape must have been interfering with the operation of the thumb shifter.
It's still not functioning 100% correctly, but we're heading in the right direction.
Which shifters? Only ask in case they have two routes for the cable when exiting the body to the bars? Apparently Campag prefer the lower path that takes the cable towards the ‘back’ of the bars as a more efficient route. At least this is the case on my centaur brifters.