Amir and Jack - thanks for the feedback.
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I have fixed my speedo today. After removing the dashboard (again!) and checking all the connections I moved on the looking at the sensor mounting.
It's almost impossible to reach from below the car and I needed to adjust the gap between the speedo sensor and gearbox lobes. I couldn't face the job of removing the whole gearbox tunnel so I cut a hole in the side which will be plated later. The manufacturing instructions say that the gap should be set at 1-2mm and I had set it up at 2mm when I installed the sensor. Reducing this to approx 1mm fixed the problem.
Speed Sensor adjustment by
Sabrebuilder, on Flickr
With 4th Gear selected the speedo is registering just over 10 mph at idle. I seem to have clocked up 1.8 miles already during testing. Final calibration will be done once I have selected my tyre size which is still TBD.
Speedo working by
Sabrebuilder, on Flickr
Happy days.........
Peter