
For a start, I've had no trouble with my door, which I think is of the later pattern and has a different lock mechanism than yours.
Could it be that the "horizontal shaft" (referred to as cable or pullrod in the respected mr. Haynes most excellent service and repair manual,) is causing the rear lock mechanism to reach its limit of travel before the front lock has moved sufficiently to release? In other words it is out of adjustment.
The instructions are a bit difficult to understand, (told you I was dim)

Mike