The Nicky Henderson-trained eight-year-old was one of three horses allotted top-weight in the fiercely-competitive handicap, although leading conditional James Bowen claimed a very handy 5lb.
There were still several horses in with a chance jumping the second-last hurdle, but approaching the final obstacle it was between 7/1 shot William Henry, Red Indian and Spiritofthegames.
William Henry produced a big leap when he needed one to to take a slender advantage and found plenty for pressure on the run to the line to score by three-quarters of a length.
Spiritofthegames was a clear second, while Red Indian just held the fast-finishing Topofthegame for third.
Bowen said: "I can't thank Mr Henderson enough for giving me the opportunity to ride him. I thought he had a very good chance and he has gone and done it.
"I've done plenty of schooling with him since his last run over fences. I think he might have just scared himself a bit that day. He jumped really well today and there is no reason why he can't go over fences again."
He added: "I can't believe things are going as well as they are, it's incredible. Hopefully it will keep going."
Henderson was not present as he was marrying his partner Sophie in Scotland.
Toby Lawes, his assistant at Seven Barrows, said: "I've not heard from Nicky since this morning. He should be all married and done and he is probably just having a good party now!"
Of William Henry, he added: "He was fantastic. Last time at Cheltenham it really just knocked his confidence. From the start it was not going right and we made the switch back to hurdles.
"He showed so much ability over hurdles last year. He came into this off a mark of 145 and James Bowen is a brilliant conditional jockey taking 5lb off his back, which is invaluable. He rode him beautifully and had him in a fantastic position exactly as instructed - bang on the rail and just sneaked his way through.
"James got off him and said he jumped so straight and with such confidence you wouldn't put off having another chance over fences, but I don't know when that would be. Firstly we will just have to assess what they do with the handicap mark and have a little think."
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