Norton F1 Sticky Choke.

Hi again everyone.
I have just worked out why the Rotary is running like a hairy goat.
The choke seems to be stuck in one position, basically half on.
After removing the tank I can see that the plunger (?? brass pushrod thingy that is conected to the cable ) is stuck.
Its difficult to diagnose as the Plenum chamber covers the view of the carbys.
Has anyone ever had a seized or stuck choke system?
I have manually pulled the choke cable and the lever mechanisms are jammed.
Cheers Mark