The firm said they were taken by surprise yesterday with the arrival of a £10,000 cash wager in a Cornish betting shop.
Ballytrim was a 40-1 chance with Hills before the monster bet and was halved in price to 20-1, after they took on the £400,000 liability.
The ten-year-old has already shown his staying abilities with a victory over the marathon distance of 3m6f at Punchestown two years ago and was last successful at Naas last March.
He beat Bluesea Cracker at Naas and that mare later went on to land the Irish National at Fairyhouse.
Ballytrim’s latest outing saw him finish third behind Siegemaster and The Midnight Club in the Thyestes Chase at Gowran Park.
Kate Miller, spokeswoman for William Hill, said: ‘It’s not very often you walk into £10,000 for 40-1 chances and even rarer to see them in a conundrum of a race like the Grand National. It is possible the whole county of Cornwall might have clubbed together for an epic West-Country coup!’ Ballytrim is owned by Violet and Archie O’Leary from Cork, best known for their association with Florida Pearl and, more recently, prolific winner and Cheltenham aspirant Golden Silver.