Marquand discovered he would be riding Joseph O’Brien’s colt barely 24 hours before the big race – after regular jockey Shane Crosse became unavailable because of a positive test for Covid-19.
While Crosse must therefore begin his period of isolation in Ireland, Marquand took his place and prevailed by a neck at 4-1 from 16-1 shot Berkshire Rocco, trained by Andrew Balding.
Pyledriver appeared set to make a bold bid for glory before wandering from a true line out on his own on the far side, eventually finishing third, a length further back.
Just behind in fourth was Santiago, who was sent off the 5-2 favourite for Aidan O’Brien and Frankie Dettori.
Galileo Chrome provided jockey Tom Marquand with a remarkable first Classic victory in the Pertemps St Leger at Doncaster.
Marquand, who earlier this year lost the ride on English King to Frankie Dettori before the Derby, said: "My heart goes out to Shane Crosse.
"I can't say how bad I feel for him because we've all been in situations where things haven't gone our way and we're both relatively young, so I can relate and he'll be sat at home in pieces, no doubt.
"I guess in racing it all comes back round. No doubt he'll have his time and I look forward to seeing him do it."
He added: "I was told I'd got the ride on this horse just before I rode Sacred in the Flying Childers on Friday.
"It really is a dream come true. Classics in Britain are some of the hardest races to come across. Group One races in Britain are equally hard.
"To have my first Group One winner on UK soil in the St Leger for Joseph O'Brien, who when I as growing up was one of the best jockeys in racing and is now training and doing a similar job - it's mind-blowing to get an opportunity like this."
O’Brien, watching from home as a Covid-19 precaution rather than attending Irish Champions Weekend, voiced his congratulations to Marquand – and sympathy and encouragement for Crosse.
He said: “It’s fantastic, a great performance from the horse – very tough, genuine and stayed very well. Tom gave him a fantastic ride.
“It’s great for Tom. He’s obviously been riding extremely well for a number of years now all over the world – and when he was available, we didn’t have to look any further. It’s very well-deserved.”
As for Crosse, he added: “Shane, I’m sure, is obviously gutted to miss the ride on him.
“But Shane’s a young man, and he’s a very talented rider – and he’ll have plenty of big rides in the future.”
O’Brien was himself adding St Leger victory as a trainer to his success when riding Leading Light at Doncaster for his father Aidan seven years ago, having come within three-quarters of a length of landing the Triple Crown with Camelot in 2012.
Unable to witness the action in person on the track, he said: “I’m at home today – Shane obviously had been in the yard during the week, so just as a precaution any of his close contacts are in the process of being tested, and I just haven’t gone racing to err on the side of caution really.
“But I’m enjoying the racing! I’m lucky enough to be able to watch it from home.”
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