Car died and won’t restart

Hi. I’m having major issues with my 2007 RR Vogue SE tdv8. I was driving home a couple of weeks ago and I started getting warning messages on the info screen, they pinged up one after the other, HDC fault, air suspension fault, EAS fault, terrain response fault, limited gears and then engine fault, then the engine cut out and I coasted to the side of the road. I tried restarting the car and it did start for a few seconds and then died again. The central locking and interior lights all work fine and so does the radio. When the key is inserted into the ignition you can hear the key lock soliniod dissengaging and I can turn the key nonproblem but when you put the key into position 1 none of the normal startup lights come on the speedo but a message comes up on the screen saying key is in ignition, it’s as if the ignition system is completely dead. I called my road side assistance and they turned up and the mechanic tried all the usual stuff, resetting the car by unplugging the battery terminals and touching them together, nothing, he tried starting it using a jumper battery incase my battery was dying, still no joy. He tried plugging in a code reader but as the ignition system is dead the reader I ain’t getting any power from the obd2 port. Got a truck to take the car to my local garage, did some trouble shooting with the garage and discovered that there’s a little switch under the steering wheel that when pressed starts all the ignition system as if you are bypassing the key, this is called a transportation switch and is used when delivering brand new range rovers to dealerships so they can lower windows ect without the key. So i presses the button under the dash and put my key in the ignition and the car started and ran perfectly normally, no error messages at all. As soon i I release they button the engine stops and all the dash lights go out. Me and the mechanic thought perhaps it’s the ignition switch that’s faulty as the car obviously runs fine when pushing the transportation switch which by passes the ignition key system. He said removing the switch is a pain in the ass on the 07 vogue as the key in in the middle of the dash and needs the whole dash to be removed to get to it. I’m a fairly competent diy mechanic so I told him I will do that rather than pay for it to be done, he said good because he hates working in range rovers ! Haha. Anyway I hit wired the transportation switch so I could drive the car to my house, it drive perfectly home, no issues. I stripped the dash down to get to the ignition, removed it and tested the contact part which is at the back of the lock assembly, it’s a standard BMW part the same as in an X5 ect. The part tested fine so the ignition switch is not the problem here, the transponder in the key must be working as it unlocks the key soliniod when the key is inserted into the lock so I don’t think that is the problem so I’m gueesing it’s either the high speed bus having a communication fault to the immobiliser or the immobiliser is faulty. I have never been able to scan codes off this truck since I bought it two years ago as when I plug code readers into the obd2 port the readers just say link error, so there’s another issue with that part of the car which isn’t helping me diagnose what’s wrong either. If there’s any one that can shine a light on what might be going on with my RR I’d really appreciate it, I’m getting ready to tear my hair out !