The eight-year-old was sent off at a double-figure starting price after a lacklustre showing in the Ryanair Chase at Cheltenham, but relished the step up to three miles for the first time.
There were four in with a chance at the last as Planet Of Sound led Cooldine, War Of Attrition and the wayward Denman.
Richard Johnson kept the winner up to his work though and despite racing over the trip for the first time, he held on gamely from 20-1 chance War Of Attrition, who was having his final race before retirement.
Siegemaster took the field along through the early stages before Tony McCoy decided they were not going quick enough and pushed on the Paul Nicholls-trained Denman.
However, the questions about him going right-handed proved valid as the longer the race went on, the further out to his left he jumped. At one stage McCoy looked to have lost all control and it is to his credit that he emerged back on the scene with a chance in the straight.
Former Gold Cup winner War of Attrition briefly looked as if he was going to sign off his fantastic career with another Grade One victory while Cooldine was also going well for Paul Townend.
Two lengths covered the four over the final fence, but it was Planet Of Sound who saw it out best, winning by two-and-a-half lengths from War Of Attrition with Cooldine and Denman claiming minor honours.
Joncol was withdrawn at the 11th hour due to the unsuitable ground while J'y Vole fell on the second circuit.
Hobbs said: "I'm chuffed to bits. He's always been a very nice horse but he's stayed on very well today. That was his first time over three miles but he definitely got the trip no problem.
"He just made one mistake down the far side but apart from that, he jumped great. He's always looked very good but at Cheltenham (in the Ryanair] he choked so we put a tongue tie on him today.
"He's been the forgotten horse today and I couldn't believe what price he was."
War Of Attrition's owner Michael O'Leary said: "I'm delighted and that was fantastic. That is it now, we'll take him home in a week or two and there is no way he will run again."
Nicholls said of beaten favourite Denman: "Going right-handed used to be an issue, but we certainly didn't think he would run like that. It was obvious he was going left after three fences and the further he went, the worse he got.
"AP (McCoy] said he would have won a minute going the other way round, he has got to go left-handed these days, it is as pure and simple as that. He'll come back and probably run in the Hennessy in the autumn and we'll stick totally to left-handed tracks."