Turning the key 90 degrees clockwise in the driver's door barrel should mechanically unlock the door. If the key is turning a full 90 degrees and nothing is happening, then it's highly likely that the lock has been removed previously and not put back again properly.
The door lock barrel connects to the locking mechanism via a flexible shaft. When removing the locking mechanism, the flexible shaft, which remains attached to the lock barrel, moves out of position. When reinstalling the lock in the door, care and attention is needed to ensure that the flexible shaft from the barrel mates correctly with the door lock mechanism. Unfortunately, this is not infrequently overlooked.
I've removed countless driver's door locks for fellow club members in order to fix the common microswitch circuit fault. Most of the time, I find that I'm the first person to dismantle the door since it was built at factory. However, on occasions, it's clear that the door lock mechanism has been removed previously, and I find that the flexible shaft that connects the barrel to the lock mechanism is just hanging in free space. Unfortunately, I fear this is what's happened here. The driver's door lock barrel is the only fail-safe way to unlock the car should the battery die.
my mrs said she spotted somewhere on a forum that you could energise the power windows from somewhere under the car and then wire through door shut to press window button
Unfortunately, even if your wife remembers correctly what she read, it's a nonsense. The electric windows are not just a simpe analogue circuit with a switch and a motor, but are instead controlled via digital commands. There is no way of energising the windows from beneath the car, and even if you could, pressing the switch would do nothing; the car knows to ignore the signal from the electric window switches when the ignition isn't on.
I managed to get a loop round the internal handle but even pulling inner handle to full extent the door stayed locked
You did well to be able to pull the handle remotely, but unfortunately that was never going to work because the car is 'deadlocked'. If the interior handles continued to work with the car locked, thieves would simply break the window, reach into the car and open it. As such, when the car is locked, the interior handles become mechanically disconnected.
Does the passenger side latch assy have a slot for where a lock barrel would go?
No, unfortunately not. The driver's door lock mechanism is unique, as it's the only one to feature a mechanical connection to an exterior barrel. The other three door locks are of a slightly simpler design.
The easiest way I could think of getting in would be to pop the rear boot release button/switch out and then put 12v though the pins that active the boot latch.
Unfortunately, the boot latch is driven by the CCCU, not by the button. The button sends an 'open boot' request to the CCCU. If the car is locked, the CCCU ignores the request. If the car is unlocked, the CCCU drives the boot latch actuator. So, you'd have to access the wires that go from the CCCU to the boot latch actuator, and I see no way of doing that through a hole above the number plate.
I'm in Lancaster, so a little way up the M6, but not that far away in the grand scheme of things. I know the A2's electrics and central locking systems like the back of my hand. If you'd like me to visit and see what I can do, the offer is there. I also have plenty of original A2 glass panels, so can do the necessary repairs should we find that breaking a window is the only option.
Cheers,
Tom