Now found a photo of a Wellington C.XVI [ Pictorial History of the Royal Air Force vol. 2]. It's in usual Bomber Command colours. I can't quite make out the codes but the port side is clearly visible and there's no indication of a door.
The caption says that the C.XVI was a conversion from the Mk.Ic, with gun turrets removed, bomb bay sealed, seats fitted in the fuselage and - in the 24sqd. example shown - dummy turrets [ they appear to be turrets painted onto the fabric skin ].
Some of the earlier marks had a triangular window in the port side aft of the wing. I wonder if that window hinged upward Danny,and you sheltered under that?
The question of escape hatches arises - from photos and reading it looks as though there were hatches in the cockpit roof, the astrodome was a hatch and there was a starboard hatch [ although no hint as to where that was].