Thomas who along with prominent owner Dai Walters was involved in a helicopter crash in November, was celebrating his first winner of the calendar year.
Winner of the Welsh National last season, Thomas and connections have had to be patient as the 10-year-old suffered a setback.
Having his first outing for 383 days, Iwilldoit travelled incredibly well in the hands of Stan Sheppard.
It was Threeunderthrufive, one of only two in the race carrying more weight than the winner, who hit the front going down the back straight for the final time - but Sheppard was still sitting pretty.
He sent him to the front turning for home and looked a class apart as he soon put distance between himself and the others.
Willie Mullins' Mr Incredible stayed on for second, beaten two and three-quarter lengths, with Notachance third.
Racing off a 7lb-higher mark than that which he won off at Chepstow, the 12-1 winner now looks a real Aintree contender.
Paddy Power introduced him at 25-1 for the Grand National.
"We didn't get him in till a bit later unfortunately and he missed the Welsh National. But he's eating well and we can train him a bit differently now," said Thomas.
"Before the Welsh National last year he was hardly ridden. I'm just delighted for everyone. It's emotional.
"He's notoriously been a fussy eater and a worrier, but as the years have gone on he's matured and knows what his job is - he's certainly easier to train than he was.
"They are his conditions (heavy), the speed he was going at the start was the same speed he was going at the end, he's very one-paced. It's nice we can come and have a go in these big races, the softer the better for him and it is nice to see he is still able to win off that mark.
"He's a star, he's very genuine. You won't find another horse to try like him - it's very straightforward when then want to do it just like him.
"Credit to Stan who gave him a great ride and well done to team and Leah (Dix) who rides him every day. That's what we do the job for and it is nice we can come here and take on the big boys and have a go.
"We had a good start to the season, from November onwards for obvious reasons (the crash) we struggled a bit. It's not an easy game and you need these big winners to keep the wheel turning."
Of an Aintree bid he said: "I don't know, he needs another run over fences, he's still not qualified to run in the Grand National. It's been a whirlwind season so far, so we'll enjoy this today and go from there.
"Maybe the Grand Steeple-Chase (French Grand National) in May would be an option. That's what I was thinking before I came here today and you would get heavy ground. You need a Gold Cup horse to win that, but he could go and pick up some prize money.
"He would have to get an entry (for Aintree) though."
Sheppard, who made it a double on the day when winning the last, described it as one of the biggest days in his career, with most of his other big-race glory coming behind closed doors while Covid restrictions were in place.
He said: "It would be right up there with the Welsh National and Betfair Hurdle, it wouldn't be far behind them and there were people here this time, so it slightly nudges it up past the Welsh National perhaps.
"It's a fair training performance that is. I don't know what he will do now as he isn't qualified to run in the National, but to win that off 147, you are probably looking at 150 plus now and the Midlands National off that looks tough."
On his celebration when crossing the line, he added: "There were no people at Chepstow and there was today. I know how much it means to everyone, they put all the hard work at home and I just get the good 10 minutes."