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 :-)

Thanks. I'll have a look at that tomorrow.
Why would the fault correct itself after the engine had been turned off for
a few seconds and then restarted?
(BTW: Is there an easy way to see if the flap is stuck?)

Thanks, Colin