Well I got my Disco back today and I'm pretty sure Michael (Service Manager at Solitaire) has solved the problem. They found that the the top of the fuel tank had caved-in due to too much negative pressure causing a vacuum. Most probably caused by the filler cap supplied by the auxillary tank manufacturer. They fixed the tank by putting in compressed air and it popped out like a brand new tank. The caved in tank was most likely the cause of the faulty gauge reading as well as a computer glitch not detecting a "full tank" when it should have. On long trips I would pump the fuel from the auxillary tank to the main tank without stopping so the vacuum in the tank would just get greater and greater.
Michael supplied a Defender fuel cap which breathes a lot better than the after market one (at long last I have at least got a bit of a defender). He has also contacted the auxillary tank manufacturer who were very happy that he contacted them - they are going to investigate.
I am very impressed with Solitaire - they went the extra distance and did not charge me plus they loaned me a new Discovery Sport for the period (so smoooth to drive). Phoned me up every day to let me know what was going on.Great service.
Thank you to the forum members for their input and to Solitaire.
Some photos of the tank attached:Attachment 120941Attachment 120942Attachment 120943

