Jumps jockey John Allen has claimed a milestone Group One win in the South Australian Derby at Morphettville aboard Howard Be Thy Name.

An Irishman who came to Australia five years ago, Allen is a stable rider for Darren Weir in Victoria and the leading trainer regularly uses him in flat races as well as on his jumpers.

With Brad Rawiller in Brisbane riding for Weir, Allen was given the chance on Howard Be Thy Name in Saturday's 2500m staying test in Adelaide.

Godolphin's Etymology had to settle for another Classic second to go with his runner-up finish to Tarzino in the Victoria Derby as Howard Be The Name went to the line strongly under confident riding from Allen.

"I came fron Ireland as a battling jumps jockey," Allen said.

"I never rode on the flat over there.

"I was grateful to get the ride today and happy to get the job done for Darren. It was a messy race early but he was always travelling well.

"All credit to Darren."

Weir was at Caulfield on Saturday where he had three winners and said the Derby victory was a thrill.

"It looked a messy race but Johnny held his composure and rode him terrific," he said.

"It's a big thrill.

"He (Allen) deserves it. He's a great rider."