Mark
I am working on a 408 which should be much the same as the 411. If it is, the fuel tank is behind the rear seat and below the parcel shelf. From recollection, I removed the parcel shelf, which revealed the top of the fuel tank. There are 2 "removable" items on the top. One is the Fuel Reserve Unit and the other (I think it is round) should be the float unit. If you have a problem, let me know and I will send you a photo (my bits are currently detached from the fuel tank which in turn is detached from the car!).
The relevant Spares Handbook page can be found here:
Resources - Bristol Cars - Owners and Enthusiasts Forum
George