Yes I was able to cut the wire and then used needle nose pliers to carefully pull it out where it was stuck in the cable mechanism. This seems to have fixed the action of the mechanism.
Ive since tightened up the cable to the front derailleur and while I haven’t put a chain on yet the lever seems to move thr derailleur cage to the right positions over the crank both outside and inner.
So fingers crossed we’ve licked it!
Many thanks to everyone!
Carry over question - does anyone know what year was the last these side-exit cable Flight Deck STIs were used before switching over to all under the bar tape cabling?