Well, it arrived today, and I had a few fiddly issues, all of which have now been resolved and my speedo is now driven from a sender attached to the gearbox output.
Issues:-
1) There are three wires on the sender, a + a - and a return (generated signal). The acewell has 2 wires which when probed apear to be a +5 V and a return for the switch. After many combinations I found the follwoing set up to work. Connecting a fresh new +12V (anything between 5 and 13 is allowed) to the + (red cable on the sender) then connect the black - cable to the white return to the acewell, and connect the white output cable from sender to the Green (what apears to be a +5V) on the acewell.
2) Wheel size / gearbox revolutions. I started the mathematical way and using a pointer and varous markers rotated the rear wheel until I could found that a Multimeter recording the output on the white wire from the sender jumped from 0 to 5 or from 5 to 0 (there are 4 pulses per recolution from the gearbox) I measured these and found that the wheel rotates 1.174 revolutins to every full rotation of the gearbox output. From this I calculated the circumference required for the acewell was 337mm. I think my measurements were not too acurate, but it was a good starting point, after attaching the satnav I found a setting of 310 to be more accurate. (Note to myself. If I change wheel size I need to devide the circumference by 5.092 to get the size needed for the acewell)
3) The fittings do not match the A series gearbox, I wasn't sure that they would, but using an old spedo cable I soon had it fitting properly. The screw cap is the wrong pitch, However a small screw removed from the sendor allows the cap to be removed, a quick snip through the old speedo cable and off comes the old cap, this gets put on the sender , and a jobs a good un. Next the square probe isn't the right size square, it is too small and the is a bit too much play. I too an old 5mm bolt, and filed it to a square and swaped out the probe (the probe is interchangable anyway).
If anyone else is looking at doing this they may wish to opt for the type that can be put onto an exisitng speedo cable as that will work with less messing about than I did. I am however happy with the stability and steady speedo readings I am now getting.