A three-time winner earlier in the season, she was then defeated in Grade Two company at Warwick last month, but roared back to form with a brilliant performance from the front end.
A superb leap at the last sealed victory for the 40-1 chance, who is a first Festival success for Gold Cup-winning trainer Kim Bailey in combination with training partner Mat Nicholls.
The latter said: “You’re going to have to give me a minute, I’m a bit excitable!
“She’s just improved all year long. We thought long and hard about whether we should come here or not and we just thought with it being on the New course it was a bit of a stiffer test for her.
“Tom was absolutely brilliant on her at the start. It’s a shame the starts this week have been a bit of a shambles, but Tom Bellamy at the start there was an absolute genius – he has somehow managed to get to the front and I still don’t really know how he did it.
“I thought she’d run well – I thought she might get in the first six, I can’t say I thought she’d win.”