Hi there, you might have a problem with the fuel sender inside the fuel tank - these were fitted with a plastic barrel and sometimes they seep in fuel and therefore are heavier because they are not properly floating.
When this happens, the fuel sender will indicate a lower amount of fuel in the tank than you have actually got which means you get the red flash indicator of nearly empty!
To rectify this, you need to remove the fuel sender unit from the back of the tank to check if it a barrel problem. If you can get your hands on another sender unit you can connect up the wiring to see if another sender unit gives you the right indication on the dashboard.
A replacement fuel sender unit is about ?70 (got mine from Paul Matty) but you must be careful to have minimal fuel in the tank before you remove the sender unit!