Mr MPuk...Lousy Transit!!!!?

I have a 51 reg 2L td transit, it keeps breaking down. I have replaced the fuel filter housing and filter as suggested but it keeps doin it. i checked the filter for metal filings, didn't see any. it'll drive fine, then conk out. sometimes for 200 miles, sometimes 20 yards. I have removed the fuel filler cap when it has broke down, nothing hisses but when i remove the fuel line from the tank at the fuel filter, air gushes into the filter (its under a vacuum to me). i then blow down the line into the tank and hear bubbles in the tank, reconnect, bleed and after a few turns it fires and goes again. i have noticed, if i dont remove the fuel line and blow down it and try to bleed it normally, it doesn't do anything. Had it in to the menders explaining everything and they have found faults with the crank sensor (!) and the fuel pump (that would be obvious as it will be a fuel pressure sender and its not getting fuel!!) they want £1200 for a new pump but being an oiil engineer they dont make any sense. I'm sure its either the fuel pickup gauze blocked or a faulty non return valve from the tank. Any ideas anyone? Any help greatly apreciated!!!!!!!