Tom Gibney's 33/1 shot, under Andrew Thornton, held off Out Now to win by four and a half lengths.
The pair hit the front over the second last, taking over from long-time pacesetter Deal Done, but it was the 10-year-old who had the stamina at the end of the three mile, five furlong trip to take victory ahead of the 14/1 chance.
The Cheltenham Festival scorer Alfie Sherrin (10/1) was back in third ahead of Paddy Pub.
Gibney said: "I only have the five horses and this is unbelievable, just brilliant.
"The last day was the real shock and we were confident coming here but we were afraid to say it.
"Andrew is a brilliant rider - much under-rated and he gave him a copybook ride, the whole thing was copybook. He is only a pony and anyone could ride him at home."
Thornton added: "That's absolutely unbelievable and full credit to the horse.
"He is tough and he jumped like a stag. He is very gutsy and never put a foot wrong. I was squeezed for room at the fence in front of the stands but otherwise everything went great through the race."