Hi Huw
Hopefully you’ve solved the issue, but have you had a look at the parts list on this website to check the dimensions of the cable?
I know the hungarian models did have some minor alterations – and some markets had a stop switch on the front brake backplate, so perhaps there are other differences on the backplate. Maybe that’s why the inner cable may appear to be too long? It wouldnt explain why one cable is fine, and the other isnt!
Have you got the dimensions of the old cable? I have a box of NOS cables which I can go through and measure up.
I fit venhill cables by default on front brakes. They have less friction and thicker inner cables, so give a better brake. Perhaps try them. They may be able to help with a custom cable, should you find the standard cable is unsuitable.
Ollie