Andy,
How do you tell if it's a genuine Caterham item? Do you know what they look like?
The only other long range tank i have seen in a Caterham, is the race fuel cells which the endurance Caterhams used and cost over £1k.
Mine is an Ali tank, which looks like a double height Caterham tank, is that what you are after?
If you know how to tell if it's a genuine Caterham item, let me know and i will check, as i haven't looked at the tank for over 6 months.
Cheers
Daren