As per Mr Pastry's comment, I can put slits in a few washers and bung them in the post if you like? I should be able to get them out to you tomorrow, just let me know which size that washer is (I'm guessing that it was in one of the bags I posted you last week).
@Mr Pastry is this what you were thinking of? (Microsoft Paint FTW)
Thinking back to brake cables on bicycles, they are usually run straight and any curves are taken as gently as possible. The loop/bend under the car and the slack near the carb end won't be helping things. It might be prudent to trim down the outer cable, this comes with a few obstacles challenges; chiefly getting access to cut the outer at the carb end (as that is the only end that can be trimmed). The cable could be disconnected from the carb and pulled out from under the car. IIRC the handgrip end is easyish to undo, meaning that the whole assembly could be trimmed off the car. I reckon that I could find a bench mounted vice at work that I could 'borrow'. A few minutes with a junior hacksaw would give a better fitting cable. The inner could also be removed at that time and lubed and inspected.
I'm wondering if I can get up to help again during the Christmas period if you are free? Once the pre-Christmas madness at work calms down, I should be able to escape for a bit. It depends on Mrs Boom looking after the kids, but it could be done if you would like a pair of hands.
Also, is the choke cable AWOL? If the run is straightforward it could be done at the same time. Not sure where your choke controls are (they vary on Invacars don't they?)
You're on the cusp of that chip shop run now!