Having got a replacement cable from another Superspec owner I decided the only way to connect it to the gearbox was to put the car on a lift to give proper access. So it was round to my friendly mechanic and put her on his lift. Even then it was a bit of a struggle. But now it works.
As expected, it is over reading drastically, almost 25% when I compare it with my GPS speedo.
The reason for this is that the gearboxes were destined for the Ford Granada so were fitted with a 22-tooth speedo drive. It appears we actually need a 26-tooth drive (although some people tell me that 25 teeth is the most that Ford produced) to get the correct speed and these are practically unobtainable.
Just have to keep looking.