This is where you need a priming bulb and I bet the Mondeo hasn't got one! If you can engineer a priming bulb (EuroCarParts sell one) into the fuel line before the fuel filter, this would show immediately if there's fuel up to the filter. A common fault on the Fiesta was tiny splits in the plastic fuel pipe which allowed fuel to run back to the tank. When this happened, the priming bulb would be felt to be empty. With some faults, squeezing the bulb would bring fuel up and the car would start, and with other breaks, you'd hear yourself blowing bubbles in the fuel tank...