By Neil Smith
Entertainment reporter, BBC News
Doctor Who star David Tennant and his latest assistant Catherine Tate have attended the launch of the BBC show's fourth series in central London.
Neither would be drawn, however, on their future in the programme beyond the three specials planned for 2009.
"I want David to stay on forever," said Russell T Davies, the show's executive producer and chief writer.
When quizzed about his own future with the programme, Davies told reporters it was "none of their business".
"I'm committed as long as I'm committed," continued the Welsh writer, the main creative force behind the long-running show's 2005 reinvention.
"Never mind what happens behind the scenes, it's what happens on Saturday night that's important."
"I don't think Russell is about to leave," said Tennant. "I think he's about to start writing the specials."
In the first of two episodes screened on Tuesday, the Doctor was seen being reunited with Donna Noble - Tate's character from 2006 Christmas special The Runaway Bride.
The first episode of Series Four sees them reunited unexpectedly
The second saw the pair transported to ancient Pompeii on the eve of Mount Vesuvius's eruption - impressively rendered in a computer-generated sequence Tennant called "gob-smacking".
The actors were joined by several cast members from series four, among them Peter Capaldi and Tim McInnerny.
Former Doctor Who baddie Simon Pegg was also in attendance, alongside Elisabeth Sladen, aka one-time assistant Sarah Jane Smith.
Pegg, to be seen next year as the young Scotty in the forthcoming Star Trek prequel, said the two episodes had been "wonderful".
Future instalments will see the Doctor meeting Agatha Christie, confronting classic villains the Sontarans and catching up with his former assistant Rose, played by Billie Piper.
Asked whether he had wanted Piper's return to be kept secret, Davies said it was inevitable her comeback would be revealed prematurely.
"It's a nice bit of buzz and there's lots of excitement, so there's no harm in the end."
Sarah Lancashire plays the villainous Miss Foster in the same episode
He was less accommodating, however, when asked about Doctor Who's new time slot - 1820 BST on Saturday evenings.
"It's not a time slot I agree with," he said. "I'd rather it went out later [at 1900].
"But we've been told that it's a gateway to the whole Saturday night. We'll see on Monday morning what it's like."
Unlike Rose and her successor Martha Jones, played by Freema Agyeman, Tate's character has no romantic feelings for Tennant's time-travelling hero.
"Donna knows he's an alien," joked the comedian and actress. "She knows he's got two hearts and she doesn't want to see what else he's doubled up on.
"There was so much sexual tension off-screen, it would have been boring to have to watch it as well."
"We've had a love story, we've had an unrequited love story - let's have a non-love story," remarked Tennant.
Writer Russell T Davies declined to comment on his future plans
"The Doctor's very different with Donna than he was with Martha or Rose. I think it sets the series off on a whole new dynamic."
Tennant also promised the season finale would be "the biggest, boldest, maddest, saddest, most exciting story we've ever done".
Tuesday's audience was treated to a sneak peak at future episodes that included a brief glimpse of a Dalek.
Asked if this heralded the return of legendary villain Davros, however, Tennant and Davies remained as tight-lipped as ever.
Speculation has been rife over who will play the Doctor's nemesis should he return to the show, with David Bowie and Sir Ben Kingsley just two of the names mooted.
"I'm sure we'll find someone marvellous to play him, if that were ever to happen," Tennant shrugged.
Doctor Who returns to BBC One on 5 April at 1820 BST.