Hello Normanda,
I think I have tried everything. Seperate bench supplies, Heavy wires, inductive filters in the supply lines, large resevoir cap 10,000uF.
The system is working now. The spikes on the supply was caused when the L6203 was being switched on and off, (pulsed driven).
I now only read the adc during the period when the L6203 is not driven and the results so far appear to be acceptable.

Many thanks for your reply especially the hint on screend cable to the pot and everyone else who offered their help and advice.

Many thanks to all
Paul