Let's try and separate possible immobiliser problems from possible battery problems. What sort of immobiliser? I can never remember which 7s have which type, add a photo if you don't know what it is. Does it have a battery in the fob? Do you have the instructions for resetting?... What type of battery?} Any master switches? What's the battery voltage with everything off? When you try to crank does anything happen? Any sounds, and does the battery voltage drop? If there's any doubt about whether there's enough smoke in the battery you could jump it with fat leads and good contacts to another car with the engine running at 3,000 rpm and see if that makes any difference.Jonathan PS: "... flat as the proverbial witche's..." ... I've never heard that before, unlike "dry". But I see that you're not alone... I wonder how recent it is.