Hello Steve,
You are correct. The Opto-isolators are grounded to the ouptut side. Error in drawing up the schematic. I am testing using 2 bench power supplies, 5v,1A and 30V,1A at the moment but I do take the point about the caps.

Scoping the circuit shows that the spikes on the supply lines are in line with the rising and falling edge of the driving PWM signal. Seems as though the mosfets within the L6203 when switching are causing my problem.

There is a fudge I've looked at today. If I ensure that the ADC does not do a read untill the pulse has driven the motor (1ms On, 9ms OFF) then I should not read any noise on the pot. Wait for the L to H then the H to L (1mS driving pulse), pause a few millisecs and then read, having 9mS to take a reading before the next 1mS driving pulse.
I sure this will sort out the ADC reading but I'm still unhappy as to why I have not worked out how to stop the H-Bridge from spiking the supplies.
Many thanks to all for advice so far. Much appreciated.
Cheers
Paul