Another forum member had issues with his fuel gauge, turned out to be a loose connection. Located on RHS of bike under the fuel tank, green in colour..have a look you may be able to see it without moving anything. Anyway, it was loose, pushed together and all good. The thread is in here somewhere.
As a side note, even working correctly I find the gauge to less than accurate. Using the trip meter is a safer bet. And thats not just on Enfields. Let us know how you go