Replaced all the fuel pipes has he? Quite expensive, those quick release pipes.. It's just that I've had this very same fault on two Fiesta 1.4tdci's, a 2002 and a 2006 model. On both cars the fault was a cracked plastic fuel pipe, the one that comes round the back of the air filter to the rubber priming bulb, and the crack is caused by the priming bulb vibrating due to not being fastened tightly to the side of the air filter housing. (So the first lesson is to tie up anything which is vibrating in the engine bay!). Now, why don't you see leaking diesel if the pipe is cracked? Well, it seemed that in both cases the crack was such that fuel would flow through pushing the crack closed, but when the engine was stopped, air would seep in and the fuel would run back to the tank! Strange but true - on both cars! You have to pump the fuel back up using the priming bulb. I took the first car (not knowing what the fault was) to my local Ford dealer who spun me a story about bleed-back pipes and bleed valves and charged me a lot of money for the repair, then I picked up the car and drove off leaving a trail of diesel on the road! I was lucky to get it back to them before the tank emptied. When the fault occured on the 2006 car I sought the advice of a local mechanic. He showed me the cracked plastic pipe and said that as Ford use a non-standard, rather narrow, plastic pipe, then standard fuel pipe can be warmed up in a cup of hot water and slid over the Ford pipe. He cut the faulty pipe at the crack and slid both ends into a legth of new standard pipe which cooled as a tight fit over the original. This is the mark of a proper mechanic! Now, if I've had this on two cars, it must (surely?) be a rather common problem. If this isn't the problem on your car, then check all the quick-relese ends - and check that the water drain tap at the bottom of the fuel filter is tight!