The 25-1 shot, who was ridden by Richie McLernon, raced clear of the field alongside Berties Dream with just two fences to go.
Berties Dream fell away after the last, but Holywell just held on until the finish to win by one-and-a-half lengths to Captain Sunshine, with Jetson close behind in third.
McLernon told Channel 4: "He is a slow horse, but he travelled really well. He jumped absolutely brilliant."
Pre-race favourite Sam Winner finished well back in the field.