Two late goals made the 17-year-old Carr Hill HS sixth-former the toast of the national stadium at the FA Vase final – but the bubbly couldn't start flowing until he'd satisfied the drug testers.
After being cheered off by 3,500 ecstatic fans for turning a 1-0 deficit into a 2-1 win over Lowestoft Town, Matt was picked out at random along with Ged Smith by the FA's doping unit.
Smith didn't keep the testers waiting long – but Kirkham's young supersub was less forthcoming.
"They picked two from each team and it took me a long time!" said Matt, who snatched the winner with virtually the last kick exactly as he'd done in the semi-final at Needham Market.
"I was in there an hour or more. I drank about six bottles of water.
"You could say it was a big relief when I finally gave them what they wanted.
"After going through that it was good to get out and enjoy the night with the team and the fans."
And Matt, whose late dad Keith starred for Blackpool, revealed how he prepared for his six minutes of fame – by getting stuck into his school books.
"My mum phoned me at the hotel the night before to see whether I'd done my biology homework. I'd left my books on the coach so I went and got them in order to take my mind off the game."
Matt is sure of another hero's reception at Carr Hill tomorrow after being given Friday and today off for the club's three-nights down south.
Thousands are expected to greet the triumphant party when they arrive back tonight for a civic reception.
"It was a great feeling scoring the equaliser, let alone the winner. We couldn't have asked for a better ending to a good season," added Matt, whose exploits have made him the target for a club in Czechoslovakia.
Manager Mick Fuller revealed that he'd received an email from an agent in England inviting Matt to go there this summer for a trial.
"His mum Liz said she's bothered about him going to Liverpool University so I'm not sure if Czechoslovakia is a bit far!"
Fuller added: "His mum was in tears at Wembley and when he scored the second I'm sure he would have had a thought for his dad. His mum will be as proud as punch.
"For a boy of 17 to do that at Wembley is a dream. He probably doesn't believe what he's done – it may sink in in a couple of days.
"Perhaps I shouldn't have put him on for those last 12 minutes! There'll be a few people rattling our cage now since he's done so well."
Meantime, Walwyn explained why he'd jumped over the advertising hoardings behind the goal to become engulfed by the fans when he scored the winner.
"I had some friends in the crowd who'd urged me all week to try and do some celebrations so I thought I'd better not let them down," he said.