Okay - I managed it myself this morning. I hope there's a better way but I'll document my approach here incase it's useful to anyone else:
A) Make some space for access
1) Remove the downpipe from the manifold.
2) Support the engine from it's RHS lift mount (as viewed from the inside of the car) using a winch or engine crane.
3) Remove the single horizontal bolt on the RHS engine mount.
4) Roll the car backward which will in turn move the engine forward to provide access to the
B) Drill an Access Hole in the Tamper Proof Box.
5) Remove the Coolant Water container from the bulk head and move out of the way.
6) On the LHS face, measure 40mm down from the top, and 7mm from the rear and drill a 10mm hole (my dimensions correspond to the center point of the drill). The hole is drilled from left to right as viewed from the driver at the far back of the box. Basically the drill bit is parrallel to the crank. DO NOT DRILL TOO FAR - only enough to break through - otherwise you run the risk of drilling through the screw inside.
8) Insert a small flat bladed screw driver and rotate the screw ANTICLOCKWISE.
9) Lift the lid off.
I ended up drilling several holes to get this to work. It took me about 3 hours to get the thing off - total nightmare. If I hadn't have got to the screw I was contemplating using a hack saw to take off the lid - the metal is very soft so you just need patience (which I personally lack.)
If anyone wants pictures let me know and I'll put them up.