As most of us get ready for the summer holidays, Steven Moffat is coming over all Christmassy. Fresh from a read-through of a Doctor Who festive special, the writer reveals that the Welsh singer Katherine Jenkins is to make her first major acting appearance in it, alongside the theatrical grandee Sir Michael Gambon. It is, says Moffat, "the most Christmassy Christmas special" of the hit science fiction drama series since its return to our screens five years ago.

After his first season as the drama's showrunner, Moffat will also be hoping that it emulates the success of the 2009 Christmas special, which featured John Simm and Catherine Tate. Episode one of that two-part story had an audience of 10 million, making it the third most popular show of the festive season. In 2007, 12.2 million viewers watched Kylie Minogue guest star as a waitress on the Titanic.

Moffat's critically acclaimed first run with the new Doctor – played by Matt Smith – attracted 5.1 million viewers for its series finale, down from 9.4 million overnights for the last comparable show in July 2008. This decline has led some to wonder whether the Doctor Who juggernaut is beginning to run out of steam.

Teething troubles

Headlines such as "Sexed-up Doctor no cure for TV ratings as 1.2m desert Timelord" and "New Doctor Matt Smith is turn-off for Tennant fans", followed reports of unconsolidated figures averaging 6 million viewers for the current series, compared with an average of 7.2 million during the last series in 2008, which starred David Tennant.

Although the sci-fi series is still hugely popular with Whovians, some industry insiders have reported teething troubles for the new team. There are suggestions that the show has suffered from budget cuts – were the Daleks really redesigned for commercial reasons? While some critics have stirred up outrage over what they considered to be scanty outfits worn by the Doctor's assistant, Amy Pond, others have fretted about the long hours endured by the cast and crew. Call it the typical British disease of knocking down success, but can a show such as this stay at the top of its game for another five years?

Moffat is adamant that it can, adding that his series finale drew 6.7 million viewers and a near 37% audience share when Sky+ and other recordings were factored in. The BBC maintains that the season's consolidated average was in excess of 7 million. What is more, the latest finale was forced to compete with Wimbledon and the World Cup, making the viewing figures "extraordinary". "The BBC are massively happy with it," says Moffat.

However, there has been some public criticism. Terry Pratchett told SFX magazine that he now regards the show as "ludicrous"; and, giving Bafta's annual television lecture last month, Stephen Fry cited Doctor Who as an example of British TV's "infantilism". It was "not for adults", he said, comparing it to a chicken nugget: "Every now and again we all like it."

Moffat insists Fry is a huge fan of the show, adding: "The attempt to create an argument between myself and Stephen Fry is laughable."

When it comes to Pond, played by Karen Gillan, he points out that one report suggested there were only 34 complaints about her first appearance in a stripper outfit. "If people who had never seen Doctor Who read the reports they would have got quite excited, but it's not an episode you would have taken to a stag party. The assistants have always been quite sexy from the start and Amy just wears what young women nowadays are wearing," he says.

Following in the footsteps

Moffat admits that he can barely bring himself to think about the "extraordinary" and "vicious" work schedule. He has only had three days off in the past year (and one of those days was Christmas Day).

It was also a hard task, he says, introducing a new Doctor and a companion at the same time, and following in the footsteps of someone "as fantastic as David Tennant".

The BBC confirms that this series was subject to budgetary restrictions. Yet sources suggest this was in line with an overall 20% funding cut across BBC drama.

"We could do with a budget like Avatar's for every episode," says Moffat. But he recognises that the show needs to evolve from the CGI-tastic earlier episodes. Smith's madcap Doctor in a bow tie is a long way from the leather-jacketed Christopher Eccleston's Who in 2005.

However, many fans agree that the Doctor has changed for the better. "Christopher looked like a leading man from a gritty BBC drama because that was the context then – his clothes were sensible," says Moffat.

Nick Griffiths, the author of two books on the series, including the memoir Dalek I Loved You, says he feels that Moffat's era is "more understated and darker" and lacks the "wow factor" of Russell T Davies's shows. But he does remain a huge fan.

Adam Macqueen, a Private Eye journalist and a lifelong Whovian, says: "Moffat has been very good at disguising the budget cuts and thinking creatively – it can be a good thing not to be able to use too much CGI, particularly because Davies had got into a bit of a rut of throwing everything at these big, epic, climactic stories that actually felt a bit empty at their heart." Moffat says Doctor Who is here to stay, comparing the character to homegrown heroes such as Robin Hood, King Arthur and James Bond.

"It may be that it might have to rest one day but I think it will outlive most of the people living in Britain at the moment."