Trained by Jim Culloty, who won three Gold Cups as a jockey on Best Mate, last year's RSA Chase hero just prevailed from On His Own after an amazing run to the line.
Silviniaco Conti led over the last, but he was collared by Davy Russell on Lord Windermere (Betfair SP of 25.73), who just held the Willie Mullins-trained On his Own (BSP 24.023/1) by a short head. The Giant Bolster (BSP 18.017/1) stayed on strongly to finish third, three-quarters of a length back.
In what was a dramatic race Lord Windermere hit an in-play high of 300.0299/1 while favourites Bobs Worth and Silviniaco Conti touched 1.422/5 and 1.695/7 to win respectively.
After two false starts, Knockara Beau was quickly away, although On His Own soon took it up, while Lord Windermere was a few lengths detached in last place after a few fences.
Silviniaco Conti and last year's winner Bobs Worth looked set to battle out the finish as they swung for home, but, as that pair faltered, Lord Windermere joined in the fight late, veering to the right on the punishing dash to the line, resulting in an inquiry, as he just kept On His Own at bay.
An anxious wait followed before the stewards announced the placings would remain unaltered.
Russell said: "I can't believe it, I was never on the bridle.
"I was never in the race, the whole field was in front of me for the whole race, I had to sit and suffer the whole way. I didn't use any petrol, I went around the inside and he jumped great.
"The whole way round I was hunting, I thought the race had gone - the credit has to go to Jim Culloty, what a man."