Summer in Dumfries, and in the moat that surrounds Caverlock Castle, a former time lord is to be found in a form of transport with just a little less oomph than a Tardis.

This morning, David Tennant is stuck with a pedalo. A swan-shaped pedalo. On the opposite bank, an assorted group of technicians, tourists and Doctor Who fans have gathered to watch the actor as he attempts to keep upright on the water (there's a diver or two in there in case he doesn't succeed).

As he does so, the odd passerby stops for a moment, then turns round and heads into the castle for a different sort of sightseeing, clearly unseduced by the magic of movie-making. Still, there is a magic of sorts to it. It's the magic of movie-making on the tightest of budgets, the magic of a small group of people all pulling in the same direction. And it's the magic of a British romantic comedy that's not been written by Richard Curtis. Welcome to the set of The Decoy Bride.

It's all Jennifer Lopez's fault. Well, sort of. It goes back to a news story Sally Phillips noticed when she was looking for a screenplay idea. You'll know Phillips as an actor from Miranda and the Bridget Jones movies, you may know her from Radio 4's sitcom Clare In The Community, and you should know her as the smirking Sophie, the receptionist at the Travel Tavern where Norwich's finest very early morning DJ finds himself lodging during the first series of I'm Alan Partridge. But she's also a writer dating back to her Smack The Pony days.

Anyway, a few years ago Phillips was intrigued by the Jennifer Lopez and Ben Affleck wedding – or lack thereof. It set some mental bells ringing. "They called the wedding off and it wasn't because they had split up. The PR went on TV and said, 'We realised when we had booked four decoy venues, three decoy brides, two decoy bands and five decoy menus that what was meant to be a small, personal affair had gotten out of control.'

"I heard that and I just thought 'Decoy Bride? That feels like me.' There are women who get to be the bride – who don't chip their nail varnish, don't have strange clusters of cellulitey lumps on their upper thighs and who laugh like a tinkling glass bell and say charming things. And then there are women who seem to have an inner builder who gets things a bit wrong, who don't do being a woman, who don't do being feminine very well."

Phillips came up with an idea about a film star wanting to get married to a writer and having to find a decoy bride to fool the snappers. A decoy bride based in the outermost rock in the Outer Hebrides. Five years later she's sitting in a trailer near Dumfries as a small group of people get on with turning her idea into celluloid reality. And that is a challenge, she says. "It's really low budget. We're trying to shoot a £7 million movie for £2.5m." She likens the process to the scene in Monty Python's Holy Grail where a knight keeps wanting to fight even though he is losing limbs at a rate of knots. During the first week of shooting, the script gets bits cut off. "The script is losing locations, losing characters, losing night-time. You can't have weather, you can't have any vehicles, you can't have any helicopters, you can't have any boats. You can't have cows. I was like, 'Oh, is there going to be anything left?' So the first week was horrendous for me. And I'd lost a lot before then." Everything she had written with her co-writer Neil Jaworski had been thrown up in the air. "We'd [already] rewritten it about 40 times."

That said, things have gone surprisingly well on the shoot, she admits. "We've had some actual weather. 'You can't have mist, it's too expensive' – and then we went to the Isle of Man, and we had real mist and we had real rain. And not only that, we had real rain on the days we needed it and not on the days we didn't."

"The big benefit we've had both in Scotland and the Isle of Man is these wonderfully natural environments like this castle," adds the film's producer Robert Bernstein as he sits beside me on a bench beside the moat. "There is very little you need to do with it."

We look over to where Kelly Macdonald is standing in a wedding dress on the lip of the moat. She was, he says, the only choice they had for the title role. "She was very loyal and committed to it from the beginning." But they needed to get David Tennant (who's playing the writer) and Alice Eve (as the film star) on board too before they could green-light the movie. Such is the way of the film business at the moment. "It's very, very tough," he concedes. "Budgets inevitably have to come down because distributors around the world are not paying the same amount of money. You have to be realistic about budgets. That is just a fact. Slightly overly expensive independent films are just going to crash and burn unless they have large presales around the world."

Does that suggest on a day like today he's here to crack the whip. "There's an element of that," he says looking over to where Tennant is still messing about in his pedalo. "In about an hour's time, the whip will come out. But once it's made, we'll go through testing. With a romantic comedy you get a very strong sense of whether it works through an audience's reaction."

In the end, that's what it all comes down to. Comedies have to make people laugh. As the crew break for lunch and Tennant rushes off to his trailer to avoid the one paparazzo who's stalking the set today, Phillips explains that, if everything works, Decoy Bride will be a contemporary take on screwball comedy. "This is a modern version of It Happened One Night." Phillips is herself a great lover of Katherine Hepburn and films like Bringing Up Baby and The Philadelphia Story. The director, Sheree Folkson, is herself a "screwball nut", she adds, and Macdonald is so "obsessed with Claudette Colbert" she's even bought some of Colbert's things on eBay.

Whether The Decoy Bride lives up to those esteemed models we'll find out when it shows at the Glasgow Film Festival this week. Today, though, Phillips is just enjoying the making of it. After the chore of the writing and the horror of the rewrites, she's enjoying playing the small part she's got. "Being in it is the fun bit. Writing it, although there's a sense of achievement, it's not great fun." But on set she gets to follow Ugly Betty's Michael Urie around and practise her American accent on him – "I've been told to go for general but apparently I'm doing slightly thick Bostonian" – and generally have a ball hanging round with thespian fellow travellers. "I really like actors. I couldn't marry one but they're great company."

The director calls cut and everyone goes back to first positions. Even the swan-shaped pedalo.

The Decoy Bride will have its UK premiere at Cineworld, Glasgow, on Tuesday at 8.30pm and on Wednesday at 12.30pm as part of the Glasgow Film Festival. It goes on general cinema release on March 9 and is out on DVD on March 12