Thanks for the answer ,
yes it is the type with a magnet inside ,and yes it had come off , so I have glued it back on to the float.
With sender out of the tank , I have done a resistance check , and it rises and falls as per the location of the float .
You are correct , I have not filled the tank with fuel , and now it only has 1/3 in it ,but the gauge only just read off empty.
I thought these senders just changed their resistance as per the position of the float .
If the sender is 2" off the bottom of the tank it will never register fuel on the gauge until the float starts to lift above 2",unless I am missing something .