I actually did the moving of the position of the cable. I filed two flats on the lower groove because the diameter is larger than the upper groove and will not fit into the locating tab otherwise.

I thought I'd solved the problem, but then just as I was putting the carrier back in I noticed it was broken! The position of the small break it would seem is just enough to cause enough 'give' in the carrier to prevent it pulling the cable far enough to pull the latch on the lock.

It works because I have made the cable taut enough now, but I have to operate the handle sharply to open it, which means its borderline 'working', and the break in the carrier will get worse. So I've bitten the bullet and shelled out the £65 for a new carrier. Will be fitting it Monday!