Kika's happy being a Bad Girl .. for now

At the end of the last series her character Julie Johnston was the surprise hand that finally felled Fenner, killing him in desperation as she was faced with the prospect of being parted from her friend and partner-in-crime Julie Saunders.

"It was fitting because she was the least likely to in lots of ways," says Mirylees. "She doesn't like violence for a start, so I felt it was quite a good surprise. She has owned up to it but nobody believes her, so she has got away with it as far as I know."

At the end of the last series we saw Julie J begin to crack with the burden of her guilt. She was sectioned but, in the special Christmas episode, she returns to Larkhall just in time for G-Wing's seasonal celebrations.

Brought in with her is a mysterious new inmate who has no papers and refuses to give her name or say where she has come from. But the staff have no time to worry about their creepy new addition to the wing, a blizzard is whipping up outside and, when it gets worse, the prison loses its power.

While the other inmates start to panic, Julie J does not help matters. She is obviously still not well after her ordeal and is quite literally haunted by what she has done - she convinces herself the ghost of Fenner is following her around the wing.

"The whole episode is almost like a spoof of those clichs you get in horror movies," says Mirylees. "You're all in a certain place and the lights go out and different people start disappearing and various things happen. It's a traditional sort of story."

What was not so traditional about filming the episode was that it was actually hot and sunny outside rather than blizzard conditions. They filmed through June and July and the cast had to don fur coats and throw fake snowballs in the blazing sun.

"It was hot I can tell you," laughs Mirylees. "We had to pretend to be cold while we were absolutely sweating. But it was quite good fun in that we have to go to a prison every day, we never see the light of day, so to get out in the exercise yard was great fun."

Julie J's mental problems put a bit of a strain on her relationship with her close friend Julie S, played by Victoria Alcock. In real life, however, the two women have a strong bond. The pair met on the set of Bad Girls during filming of the first series six years ago and, like their on-screen counterparts, have been firm friends since, creating between them one of TV's most popular double acts.

"We do get on well," says Mirylees. "I have horses and she likes riding so we meet up, usually once a month or so, to go riding together. It's been lucky that we do get on so well. It would be very difficult to this job with somebody you didn't get on with.

"We laugh a lot together, that's the main thing, we have a similar sense of humour. But I'm not as much of a practical joker as her, I'm much more sophisticated," she laughs. "Thankfully she rarely does it to me. It tends to be some other wretched person in the cast.

"What does she do? Oh, I don't know, she'll put horrible things in people's drinks and do things with sticking gum. It's so childish I really can't think of any," she laughs. "I can't lower myself to even talk about it, it goes straight over my head."

Mirylees will be seeing her over Christmas as they often work together even when Bad Girls is not on, appearing on shows like Loose Women for example. But for the main event she will be staying at her brother's house in Devon along with much of her extended family.

"I have two nephews who are ten and five, which makes it lovely," she says. "Kids are a really important part of the package with Christmas, I think. You want the tree, you want the presents, you want the nice meal, the jovial adults and the kids really enjoying themselves. They're part of the whole thing, aren't they?"

Christmas wasn't always such a joyful time for her. When she was young, her mother ran away from her bullying step-father, taking Kika and her brother with her. They were constantly on the move, getting by on very little money. But still Mirylees remembers Christmas as a child with some fondness.

"We managed to rent a cottage somewhere south one year and I had a load of friends over," recalls Mirylees. "We didn't even have enough knives or forks, or a knife sharp enough to cut the meat.

"It was really cold, there were owls in the bedroom, no central heating. But we lit lots of fires and we had a wonderful time. I don't think we had electricity either, we had candles. But it was marvellous."

The coming year brings another series of Bad Girls for Mirylees, with filming set to start in February. She is reluctant to talk about how she sees her future with the show, exclaiming: "If I tell you that I want to go they'll have a fit won't they?" But you sense she is keen to try new things. "I think it will be a very much to do with what happens in the next series," she says.

"We do have time off and we can do other things, but probably not as much as we'd like to. It's nice to work with other people and it's nice to work on Bad Girls.

"You get to know your character so well you can almost tell what you're going to say before you've read the script." If she does leave, however, it begs the question - what will happen to the two Julies? If one goes, does the other one go as well?

"I don't necessarily think that would be so," says Mirylees. "It depends what the other person wants to do. Whether you're a double act or not you have your own career, but we've always talked about what we want to do and when we want to do it. We've always considered each other.

"But I've no idea. It may be that Shed Productions, who make it, feel the next series is enough. They're quite busy, with their new show Waterloo Road being filmed up in Manchester, and other projects like Footballers' Wives. They may be ready to let go of Bad Girls."

• The Bad Girls Christmas special is on ITV1 on Monday, 9pm