A first-time steeplechaser with a significant weight advantage has denied a courageous Bashboy an historic third-straight Grand National Steeplechase.

The Kathryn Durden-trained Wells, a talented hurdler having his first start over the bigger obstacles, claimed Sunday's $250,000 feature at Sandown in a thrilling contest that came down to the final 200m of the 4500m race.

Bashboy was asked to lump 74.5kg, 2kg more than he carried last year, which would have been the biggest weight carried to victory since 1957.

He stalked the leaders Sea King and Maythehorsebemagic early in the race before moving up from the 1000m mark to start his challenge.

Wells, who carried 65.5kg, was the horse Bashboy's trainer Ciaron Maher feared most at the weights in the five-horse field.

And as Wells started to work ominously into the race approaching the business end of the event it was easy to see why.

Wells ($7) and Bashboy ($2.05 fav) went to the last fence almost level pegging with Sea King ($2.70) close behind.

But Wells had the most left in the tank in the final run to the post under jockey John Allen to claim victory by six lengths over Bashboy.

Sea King, who was chasing the Grand National Hurdle and Steeple double, was the same margin away third.

"We're absolutely rapt," Durden said.

"He has promised to deliver and we've been a bit unlucky. So we're really excited."

Durden said she had always believed Wells would be a better jumper over the fences and targeted the Grand National.

She said the better than heavy ground also suited Wells who had been placed in his three feature hurdle starts this season.

Maher and champion jockey Steven Pateman were proud of Bashboy's gallant effort but said weight told in the end.

"He tried really hard and never gave up," Pateman said.

"There was a nine kilo difference and weight was always going to be the excuse.

"He was up for a good scrap down to the last fence but with that weight difference, Wells was able to give a bit of a kick and we were just grinding away."