Berlina Door Lock Problem

[nthomas1]

I have a problem with the door lock on my Series 2 Berlina.   The black catch (shown in first picture below) should turn freely.  If turned clockwise it should click twice as it engages with the first and then second pawl within the lock mechanism.  Pressing the push button in the door handle should cause the catch to spring back to the original position. 

On my car the catch requires considerable force to turn it.  And when turned to the lower position it cannot be returned by pushing the door handle button.  It stays in the downward position and considerable force has to be exerted to force the door open.   You can see filings under the catch from where it has been forced against the keeper plate when trying to open the door.

I have the door card off the car so can inspect the lock.  It is well greased.  I decided to remove the lock to see if the catch can be freed, but the trailing window runner blocks its removal - so it’s a window-out job.  The lock, of which the catch is a part, is a single part number in the TAV so probably not designed for dismantling or overhaul.  If that proves to be the case I’ll be looking for a replacement, which might prove difficult to find!

Note that this type of lock is unique to Series 2, and was introduced with chassis number 22376.  It’s shown in the detail at bottom right of the TAV page 77 shown below - the part number is 66.   The earlier design of lock cannot be fitted because a tooling change was made with a recess introduced on the door shut face where the later lock design is mounted.

I have three questions (appeals!) to the Berlina community:
1) Has anybody had a similar problem, and has found a way to free up the catch?
2) Has anybody removed one of these locks before and can provide any tips that might prove helpful?
3) Does anybody know of any source of information and tips on window removal (like the useful info that’s available for Coupes)?