Well done mookaloid, russbot and rusty nuts - you were all on the right track with your advice.
The carb wouldn't idle without racing.
There was a slight sticking sensation when the throttle was nearly closed.
When I took it apart I noticed that the screws holding the butterfly plates onto the shaft had been shifted with some force. I took the plates off and found that they are offset slightly and had been fitted the wrong way round!
Problem solved!
Thanks for the help.
Dave

great news - thanks for keeping us posted

Nice to know we got something nearly right!

Glad you got it sorted.