The 29-year-old New Zealander led from start to finish on Lucy Allison and Frances Stead’s 13-year-old, finishing on a score of 41.3.
“For Lush to win is very special as he’s been knocking on the door for a long time,” said Jock, who plans to take both him and Clifton Promise to Burghley.
Oliver Townend was second — as he was in the Smith & Williamson Intermediate Championship and also Saturday’s advanced class.
Riding Armada, he finished 10sec under the time allowed on the cross-country to complete on his dressage score of 43.1pen.
Australia’s Sam Griffiths was third on his experienced Olympic team horse, Happy Times.
William Fox-Pitt took the intermediate championship for the fifth time, riding Carol Gee and Catherine Witt’s Fernhill Pimms on this occasion. They also led from the word go, adding just a couple of cross-country time-faults to a dressage mark of 23.9.
Oliver was just behind him on Cooley Master Class, while Andrew Nicholson took third on Teseo.
Bettina Hoy was a popular winner of the British Eventing Novice Championship on Seigneur Medicott. The pair added 2.4 cross-country time-faults to their 26.2 dressage and clear showjumping to finish three marks ahead of Aoife Clark and Fernhill Adventure.
Paul Tapner was third on Pioneer Milly.
The advanced class was another success for William Fox-Pitt. He won on Christopher Stone’s Chilli Morning and finished fifth on Running Order. Oliver filled second on ODT Ghareeb, while Emily King took third on Brookleigh.
Steady rain didn’t blight the event, which fell victim to the weather in 2012.
“It was amazing how good the ground was, even towards the end of the Open Championship, considering the rain, the terrain and how many horses ran on it over the weekend,” said Jock Paget.