Some progress has been made! Thanks for the tip about using some hook like tool to pull the bottom metal bracket of the HU I used a pair of 90 Angle Cir-clip pliers to hook the unit out and with my improvised 2 din removal tools (4 small screw drivers shoved in) the unit came out easy.
In past few days the sat nav has given an initial direction from starting the car up but doesn't give any more after the 1st turn left or right instruction and then just gives me the compass pointer.
Here are a few more photos.

Photo of the Diagnostic of the BC taken with the engine running. This screen varied from 87% to 100% with engine off. Didn't take any photo while driving but none of the figures changed other than the 87%. The fuel consumption and speed all made sense but maybe the Sat Nav isn't getting some data it needs.

Photo of top of unit

Side of unit. No idea what all the stamps are about.

The other side of the unit

Back

Slot in dash unit goes in.
Everything looks fine really and ill refit the unit later or tomorrow and see if Sat nav is back.
Also managed to work out that one of the Tweeters is broken after swapping them round. Car has Bose are the tweeters something special or will any Omega tweeter do? I have a door from a front passenger door from an 99 Omega CDX that just happens to be laying around any chance the tweeter behind the Wing mirror cover will work?
How easy is it to remove the GID. I am thinking if the fault is with the HU then ill need to swap both and then I might have a cunning plan for an upgrade to a CID.