I think Paul7755 has summed it up nicely there...
The door is locked when the driver is driving the DMCL part of the train... But when the roles are reversed, that door does work...

My only slight issue with this is that when your on that 22:38 on a Friday Night, entering and exiting First Class does attract alot of unwanted attention... The Eyebrow Raising I could put up with...

The comments aren't so fun...!
