Like it says my Mercury is reluctant to run after starting since it's
winter break. Same thing happened last year and I paid someone to fix
it - it was the choke. It starts but then gradually struggles more and
more as it warms up.

Looking at what is going on in the carb/throttle/choke/idlespeed mess of
bits it looks to me like the choke valve is totally jammed closed as I
cannot throttle up fully (the choke pivot/lever stops the throttle
moving beyond the point where it should open the choke valve as it
throttles up).

As the people who fixed it last year are the only people near us I'm not
keen to take it back to them and have them do a half job on it again.
Is there anything I can do to free it up?

TIA