All the information on the interweb talks about the odometer failing due to damaged gears, but my problem was that both the odometer and the speedometer didn't work, consequently the cruise didn't work either. I checked for dry joints on the PCB, cleaned everything I could find, no improvement. I've lost count of the number of times I've had the cluster in and out of the car and dismantled it.
Finally I came to the conclusion that the problem must be internal to the speedo head itself, I really didn't want to change this as I wanted to keep the mileage correct. After much more internet scouring I found an item on correcting the mileage, so today I have fitted a replacement speedo with exactly the same mileage as the one I removed.
Success at last!