False alarm - there is water in the system - must be an old drain
cock.

Tried again today with the red wire feeding the pump switched out and
the valve manually turned on but it blew. Valve obviously kicked in
cos when I checked after the blow the vavle had retiurned to the auto
position. Tried with the valve feeds all out also also blew. Did
manage to get it going for a short while but then the boiler went out.
Managed to restart the pilot ok.

I've given up!!!

Charlie