I had the same problem in my 89 XJ40, I found that the fuel return line in the tank was not connected to the reserve chamber .
there is a chamber in the middle of the base of the tank that holds the fuel ready to be sent to the fuel rail. round, about 20cm high and holds around 1ltr of fuel. All fuel returning from the engine fills this chamber first and then pumps it back through the fuel system.
If the tank get below 1/4, there is a possibility of this reservoir running out and no fuel being pumped through the system, my return line was just lying in the bottom of the tank. it looked like it was never connected.
maybe this was a common problem with the xj40, it was a real bugger to re-connect.