Hi Mike,
I posted a similar problem both here in August 2012 as Heater "Defrost Cable", and on the MB UK site.
My Cable was very stiff.....then snapped ! the flap was stuck.
I was going to fix it myself, but failed to find good procedural information, the flap was stuck, and the cable snapped.
I cannot really tell you how to fix it as I left it to Brian & Simon at Redcastle here in Cardiff,
They were replacing the Ignition switch and doing some other work for me a month ago,
so they sorted it out then - not a cheap option - but access is really horrible , and my back is not good in the winter.
Good luck
Paul