Vaillant VMW ES/282/2-5 R1 boiler not firing up?

I have a boiler VMW ES 282/ 2-5 R1

The boiler will not light, it sparks but there is no gas. I have renewed the gas valve, model confirmed by Vaillant Spain and the pcb, again with Vaillant number, but still it will not fire up.
There is gas to the valve, but it seems that the valve will not open for some reason.
The error no is F28.
One question, does the water have to be running at the tap? I have the heating control turned up full.

The boiler was fitted in my cortijo, here in the mountains of Spain before I bought it but had not been connected to gas before. It has jets for LPG , I am told, which I am using. I have water pressure of over 2 bars and both HW and Heating turned fully on with the contacts 3 and 4 bridged as I do not have a thermostat at present.

I will be pleased of any help you can give.
Thanks in advance.

Graham P
October 2011