Jamie Moore, fresh from his win on Sire De Grugy in the Queen Mother Champion Chase at Cheltenham, steered Peter Bowen's outsider to a famous victory.
But Moore had a scare on the run-in when Al Co, having his first run for 104 days, spooked and jinked to his right but the partnership was soon back intact.
Last year's winner Godsmejudge (16-1) was a gallant second, a length and a half away, with Trustan Times (10-1) just a neck back in third and Merry King (50-1) fourth.
Moore said: "Peter told me he had him spot on, but twice up the run-in he spooked at boards or something.
"It's the first time he's run for a while and I'm delighted to ride a big winner for Peter.
"I hit the front soon enough, but you know Peter's horses are going to run to the line.
"I honestly nearly fell off twice and thank God I didn't.
"He's have gone again if they'd come to us properly."
Of his highly successful spring, Moore added: "Peter puts me up a lot and it's nice to do it for people you ride for."
Bowen said: "He was in the best form we've ever had him.
"Luckily, we've got Ffos Las up the road and he had a racecourse gallop there and he was absolutely spot on for today.
"I didn't see what happened on the run-in. I'd got four horses to watch and it was a job to keep an eye on them all.
"I thought he hit the front a bit soon but I had a lot of faith in Jamie and it's fantastic to win this race.
"To train him for the National (at Aintree) next year would be the plan."
Alan King was chuffed with the performance of Godsmejudge.
He said: "I'm very proud of him. We said all the way through it's been a very difficult preparation.
"He's only just coming back and that was a mighty run. He'll have a summer out and regroup for next year. That was awesome."
Veteran Tidal Bay ran a creditable race under his big weight in seventh, a week on from unseating Sam Twiston-Davies when badly hampered at the Canal Turn on the first circuit of the Crabbie's Grand National at Aintree last weekend.
His trainer Paul Nicholls said: "There had been talk of Tidal Bay retiring, but he will definitely stay in training next season, as long as he is fit and well.
"He has run a good race, the ground was probably just against him a bit Sam said, but he has come back safe and sound.
"We will give him a nice break, and bring him back next season, possibly starting over hurdles at Wetherby."