Heating Problem?

I have a 2001 Seat Alhambra 115 Diesel I started to have problem with the coolant system & returned it to the garage I got it from they bled the system & all appeared ok till 3 days after the warranty ran out. Since then I have had major problems with it, firstly it was overheating so I took it to an independent garage who told me the rad was blocked so they would flush it & & fit a new stat that did not work. I returned it so they said it could be the impeller on the water pump that did not work so again I returned it where they carried out a head test apparently the head had gone they gave me a price to do this I was gobsmacked! I took the car to another garage who told me the wrong stat had been fitted so they changed that still nothing they then told me it was also the wrong pump so they changed that nothing they went a lot further into it & told me there was a small electric pump under the bulkhead that was not lifting the coolant up to the heater so they changed that nothing! They have had the car for 3 weeks & cannot come up with an answer to the problem & have told me they give in. The heaters both work fine when the car is stationary you can rev a much as you want & its great as soon as the car is put under load (they put it on a rolling road) any load is put on the heater goes cold I am a single dad with 5 kids & just cant afford to put anymore money at this but on the same hand I cant afford another car can anyone help please