Hello,
Thank you for all your replies.
Because I had to set up a Parrot Blue-tooth phone system, I removed the audio system and the speedometer cluster. On top of the speedo there are 2 white hexagon spigots, these screw
the electrical connections into the back of the instrument cluster. I found these were loose but then I slackened them off completely and removed the cluster, just to check the soldered tracking. Everything looked OK, so I refitted the cluster and tightened the 2 hexagon spigots. Put everything back into place and drove the vehicle. The speedometer worked!!
Just in case it was a fluke, I drove it today and the speedo is still working.