Thanks for all of your sugestions...
It turned out to be a simple but devious fix.
When you are purging air from the fuel delivery system, you do so with the fuel valve closed. ie with the Fuel Shut-off cable PULLED-OUT.
I found that the fuel cut-off valve has a return spring! that tends to hold the fuel valve open ( that is with the Fuel shut-off cable pushed in) Then, when you PULL-OUT the fuel shut off cable...the cable pulls against the spring action and closes the fuel valve.
However,
IF your fuel valve shaft is seriously sticky and the return spring is elderly (like mine) ...the fuel valve MAY just STAY (stuck) CLOSED !
You tend NOT to notice this whilst you are trying unsuccessfully to start the engine.
...and it tends to be an intermittant problem.
Agh!
Thanks again for all the help...
Rob