Fuel Gauge messing around.
 

When the tank is full the gauge is fine When the tank gets below 3/4 tank it fall right down to empty.
I add 30 liters and it is fine until it gets to 3/4 tank.
New gauge?

That problem sounds familiar, but not with an Aston. Fuel gauge problems plagued some Corvettes, mine being one of them. After doing some research I found that Techron fuel additive was a remedy. I no longer have the Corvette, but if I recall correctly the problem was caused by deposits that formed on some contacts in the gas tank. Techron and using very good gas seemed to solve the problem. I think Chevron gas actually has Techron in it. Wishing you the best.

Check out Repants.lol blog from March 1, 2016 for " That one time my fuel gauge went binary". Rich might have some answers for you.

Sorry to hear this, highly likely its the sensor that is playing about, have you diagnosed it