The campmates had to choose two people to face the challenge yesterday, with Anne Hegerty ruled out on medical grounds.
However, despite his sprained ankle, John was able to take part, and the group decided he would team up with Fleur.
"If I have to use my cane to wade off some animals, then I will," he warned.
Fleur told him to "break a leg", to which he responded: "Thank you, I will. I’ve only got one left!"
At the Trial clearing, presenters Dec Donnelly and Holly Willoughby wondered why they'd volunteered to take part.
Nothing she was feeling "a little bit low" after missing her letter from home, Fleur joked: "I thought I’d pick myself up by doing a Trial!"
John explained: "I didn’t want people to think that because I had an ankle problem, it would stop me doing anything, because I want to stay."
'Rotten Retrieval' involved John being locked in an underground chamber, while Fleur navigated a series of dark tunnels, collecting numbered keys.
Along the way, she passed the seven stars that would earn them meals for camp - but they could only be unlocked with a tool in John's possession.
Once Fleur had unlocked John, she had to make her way back through the tunnels, collecting the stars.
She had ten minutes to complete the Trial.
As John got in to position, Dec asked: "Have you located your tool?"
"I’ve never lost my tool Dec, but I’ve found an extra one which is always good!" he laughed.
"This could be a long Trial," smirked Dec.
Fleur was showered in cockroaches, crickets and mealworms in the first tunnel, before encountering 20 huntsman spiders.
"Oh my God, a spider just injected some nastiness," she squealed.
In the second section, she had to swim through a water tank to the other side, collecting a key along the way.
Meanwhile, John complained as he was joined in his chamber by various critters, nibbling away at him.
Fleur encountered biting green ants and 50 rats before she finally reached the Torchwood star.
As she used the keys to open the padlocks sealing his cage, one particular creepy crawlie got a little too close for comfort.
"I’ve got a cockroach in my butt hole!" John exclaimed. "There is a cockroach having dinner on my butt hole!"
"They are everywhere, in every orifice that is not in my head."
Once he'd been freed, Fleur began heading back through the tunnels to collect the all-important stars.
She managed to get all seven with just seconds to spare, meaning the pair emerged from the dark maze with a meal for every campmate.
Holly asked Fleur which part had been the worst. "The rats, I hated the rats and it was so disgusting," the 'Sax' singer commented.
"The water was really scary, I kept thinking I was going to drown."
Meanwhile, Dec told John he'd been covered in 10,000 cockroaches, 10,000 mealworms and 5kg of mealworms.
Back in camp, a victorious Fleur added: "I was genuinely terrified, I feel like it was all of my nightmares thrown into one Trial.
"The jungle is all about facing your fears and getting through challenges, so bring them on."
I'm A Celebrity continues nightly at 9.00pm on ITV, followed by Extra Camp on ITV2.