Hi,
Bit of an update....

I recently made quite a long trip and so filled the tank all the way to the top. I did about 100 miles before the needle even moved and did about 350 miles at half full (ish).

The rest of the tank I did my normal driving to work and back and from the half tank left I got 240 miles or so when the fuel gauge started beeping at me. So basically just under 600 miles out of the tank before it beeped, with just over half of that in one long trip

So it seems that it is a lot more efficient (By about 100 miles) if going on one long trip and it's fair to assume I could get 700 or so out of the tank if I just went up and down the motorway all day.

Overall, not as bad as I first thought as it seems that although the needle was near the top of the gauge when I picked it up it was nowhere near full.

Thanks for all the advice

Andy