Ian-
I have the same issue, using short jumper wires at the rubber couplings does not solve it - I suspect corrosion between the inner steering column/couplings may be the issue. It is quite a pain.
Look at the column design, I am very surprise Triumph didn't make a ground at one of the column bushings - those bushings have a metal insert, in my opinion that is a better place to ground than going all the way through the steering rack. Oh well.
Randy