Danny - it gets more curious.
The book "Wellington the geodetic giant " by Martin Bowman, contains a fair number of photos and there's no sign of a door on the port side in any of the varied marks. I googled " Wellington bomber cutaway" and can see no door in the drawings shown.
A friend of mine who flew, as an ATC cadet, in OTU Wellingtons, can't recall a door in the position you mention - he says that's where the cadet passengers used to sit.
I'm hoping to get to Cosford next Sunday. The engineering hangar contains a Wellington. Whilst there's no access to it other than on fixed open days, I might learn something if I talk to the right person.
It's conceivable that the transport version had that door. I haven't managed to find a photo. of that.