Ok I found the post and it was the speed sensor. Initially the new sensor didn't work but it needed the code cleared before it would work. It had stopped working completely though.
That was on a TU3 (1.4) engine probably with a Sagem S2PM ECU.
Because it's an intermittent fault it might be logged as "pending". Or it might be a code the scanner cant read? Dunno.
If you can back probe the speed sensor connector, pin 1 is "12V", pin 2 is ground and pin 3 is pulses when the wheels are turning.
I think you can remove the speed sensor and turn the input shaft with a screwdriver to simulate the wheels turning?
Of course you can only try this when the speedo isn't working. When the speedo is working it's pointless testing anything.