The Extra host, 31, placed a twig on a piece of paper, crept up on Denise and dropped it on her lap as he said "Denise, some dog poo there."
Loose Women star Denise, 44, jumped and shouted out before realising it wasn't real and throwing the twig back at Mark with a scowl.
The star duo were joined on the outdoor set by their families, with Mark's sister Jess, 32, and mum Carol, 58, and Denise's daughter Betsy, eight, all appearing on social media.
With the group huddled together on a bench beside a campfire, Mark introduced everyone, saying: "I'm looking suave as always. It's a family affair out here."
Matalan's star-studded autumn clothing campaign also features Peter Andre, Frankie Bridge and Scarlett Moffatt.
Mark recently partied in Majorca with wife Michelle Keegan and is now in the UK as he takes a break from his TV duties in the US.
His best pal and co-star Mario Lopez has praised the couple's long-distance relationship.
Michelle, 31, stars in the latest series of the hit BBC show Our Girl and spends time with her hubby whenever she can.
She has ruled out a permanent move to America, but Mario told OK! Magazine: "I think he’s handling it well, for the most part."
Mario, 44, and his wife Courtney, 35, have children Gia, seven, and four-year-old son Dominic together.
The former Saved By The Bell star is a big fan of fellow actor Michelle and hopes she'll meet his family eventually, adding: "We’ve talked about it.
"I’ve met his wife, she’s very sweet – it’s just been tough with our schedules, hers and his, to get us all together. We will try and do that."
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