Large chunks of "Nether Zone" take place in Real Time, including the time between Jeremy ordering a pizza and the pizza being delivered. What kind of pizza is made and delivered in five minutes?
Possibly lampshaded by Jeremy when he says that "time moves at a different speed, in the Nether Zone".
At the end of Series 4, Sophie runs away from her wedding in tears, leaving Mark alone with Jeremy. At the beginning of Series 5, everyone sees Mark as a jilter. Why?
Mark hides from the first part of the wedding out of fear, for enough time to have dozens of missed calls. Couple that with Sophie's tendency to exaggerate, and you've got a recipe for disaster.
Plus, well, he did legitimately try to jilt her. He just wasn't particularly successful or competent at it. While he did eventually go through with the wedding, he clearly didn't want to but was just too cowardly to officially jilt her when he got cornered. While Mark might not be an official actual jilter, he doesn't exactly have a lot of room up there on the high ground either.
He's also the Butt-Monkey in a show which shows No Sympathy to its characters. It would be more unusual if they didn't leap to the worst conclusion and treat him accordingly.
So Sophie slept with Jez a month before Mark and then the baby was born a month early...I can't be the only one connecting the dots there. We're entirely dependant on Sophie's word that Mark is the father. It seems strange that no one inuniverse brought up the possibility that she's lying in their own minds. Something that wouldn't be out of character for her at all.
She even says that she thinks she probably made a mistake with the dates when she's telling Jeremy that there's a chance he is the father. I think she knows the baby is definitely Jez's but after weighing everything, decided Mark would be the better father, so she lied to them both.