Been a bit of a saga, long story of procrastination but another engineer has completed the repair.
Basically roll pin 17, had failed releasing ball 19. Hence the selector was unable to move around the gate 25. Also reverse pin 8 had broke, exactly when not sure but the indents on “bar” 5, were well worn. Funny reversing light was still working a couple of months ago.
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Just engine & gearbox back in car & refit tunnel & trim. I might get car back next week. The car is my every day car so has been a right pain not having it.
But could have been a lot worse as with those bits floating about could have jammed into the gears & done much mischief.

4/4 Ivory 4.1:1 axle, Mercedes A200 AMG