Thanks for the input ajme,
I have measured the coolant temp. sensor through its range from cold to hot and it follows the graph in the Mitchell on demand as regards to the resistance. But it does seem as though the duty cycle is pegged at 30%.
As posted I am now running without the eha connected and it no longer fouls the plugs, I'll run like this for a couple of days and check condition of the plugs. If they look good I will re connect the eha and see what happens.

Michael