The vacuum operated flap in the air intake was stuck down so cold air
was
always drawn in. Used to 'screw' (self tapper through the casing) the
thing
in the up position for winter and screw it down for summer running.
Crude
but effective :-)

Yippeee! Sorted. Who'd of thought it....

Looked at the flap mechanism in the air intake and it was indeed broken. The
metal wire that lifted and lowered the flap had broken off and so only cold
air was being drawn into the carb. So... even though the vacuum mechanism
was working the flap wasn't moving.

I took a leaf out of Dave's book and secured the flap open (up) using a
couple of self-tapping screws.

I have done a couple of journeys today of around 15 miles each with no
problem.

It has been a good day... Thank you to all those who offered advice.

Cheers, Colin