I got around to taking the door apart and rebuilding the drivers side door lock a few weeks ago.
For any one interested, this is what part number 51951924961 looks like.
The roll is quite heavy and seems like there is plenty for my more door

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- Part Number: 51951924961
30 year old membrane on a garaged only E30. Condition is much better than expected and the membrane gave quite a fight when I tried to remove it.
Got to use my new trim removal tools for this job. Worked a treat - didn't break any clips or damage the paint either the door card or the paint on the door.

- 30 year old waterproof membrane.
With membrane removed. The window mechanism is quite impressively over-engineered.

- Door with waterproof membrane removed.
I didn't quite get around to fitting the new membrane unfortunately.
I am also considering fitting some extra sound deadening to the door skin, however a bit torn about this at present. The door itself doesn't actually sound too tinny when tap it all over.
Closer to Christmas I am should have time to give the inside of the door a clean, spraying some cavity wax, (fit sound deadening?) and fit the new membrane.
I did however get around to using my lock rebuild kit I purchased 6 months ago... Very impressed with the updated design. Lock actual works now and have ordered a new one for the other side too.

- Door lock rebuild kit.