Hi,
I think we have the same problem! We have the same boiler and model .. also had been working for years then one day.. nothing. We have had 5 heating engineers out, had the pump changed all pipework changed and every working part checked. Still nothing, until the last engineer decided to put what he has called an oil lifter onto the exterior of our house. It now works but what an eyesore, it resembles a small lavatory cistern which doesnt look good on a sandstone listed building! The problem with us was after getting the oil to the tiger loop it was still locking out. The enginner rigged up a contraption with a bucket filled with oil and using that as the tank if you like.. which then worked. As you can tell Im no expert but I still feel the problem may be the tank, however when I saw you had the same problem with the exact same system I now begin to wonder. Have you solved the problem yet or are you still, like us searching for answers, would be interested to hear from you.
Ralph
November 2009