Barry, your bike could well have suffered a bit of damage in the tank. The fuel gauge is a piece of wire with a float on the end. As the fuel level drops the arm starts to come down. The arm actuates an electrical switch sort of like the volume control on a radio. It is possible that something may have happened, but I think it unlikely.
My bike has the annoying problem of me filling the tank up to the brim and the RID shows 1 bar missing. Normally I have to use close to half the tank before that 1 bar goes off the RID. All the way down as the tank emptys the bars go off before they should. Eventually I have the amber fuel light showing 1 bar before it should come on.
I found out that there is a possible culprit causing this little gremlin. The fuel tank has a main electrical cable coming from the fuel pump/gauge. This cable is on the right hand side of the tank. If you pull the black plastic panel under the tank off you'll see the cable and it connects with a largish plug.
This plug contains 4 pins (I think) these pins both the male and female ones seem to get dirty and don't pass the information on as well as they should. Pull the plug apart (don't forget to squeeze in the little tabs to get it to pull apart) and then clean the pins carefully. I then push and pull the plug in and out about 5-6 times and hey presto the fuel gauge works correctly again.
Now I'm not saying that this is a definite fix for your problem, but it works on my bike and also on quite a few friends bikes as well. I think it's age and useage related along with dust and whatever, but it appears that this is a problem that may occur when these bikes are 20-30 years old and everyone will know about that fault, maybe.
Mick.