After 12 years and 13 series, Heartbeat shows no sign of stopping. One of the few original stars still appearing, Derek Fowlds, admits that he didn't think the series would last beyond the first six episodes.

Now, he tells Steve Pratt, the man known as Sgt Oscar Blaketon wouldn't think of working anywhere else Heartbeat star Derek Fowlds remembers crawling through the Yorkshire undergrowth with co-stars Nick Berry and Bill Maynard in the second episode of the series and thinking to himself, "This pile of poo will never run".

He was wrong and isn't ashamed to admit it. As the series begins its 13th series on Sunday, Fowlds confesses he wasn't confident it would last longer than the initial six episodes.

"It thought it wouldn't run," he says. "It was old-fashioned and I didn't really comprehend that people were ready for this - and they were. We were really surprised.

"At the end of the first series, they were saying they wanted to do another and a director friend said, 'you're in a hit and it's going to run forever'. He was really on the button."

Fowlds's character Oscar Blaketon has left his police sergeant post behind the desk at Aidensfield nick, but lives on in the series. He celebrated his 66th birthday on set on Tuesday, and can't think of doing anything else.

"It was a magic formula - nostalgia, cars, music, characters - which happened by accident. All the things came together and it's been a joy," says Fowlds.

"I was going to leave on two or three occasions because I never really wanted to play one part. I love acting. I've done plays in London and TV's Yes Minister, and just wanted to do other roles. Each time I decided to leave there was always a very good reason to stay.

"Age is a factor. If I'd been in my 20s or 30s, I would have done a Nick (Berry, who left after five years). But I'm 66 and very lucky to be in a series that's run for 12 years."

He may be associated with Heartbeat now, but Fowlds has had two other very successful incarnations - as foxy puppet Basil Brush's right hand man and as Bernard Wooley in the political comedy series Yes Minster and its successor Yes Prime Minister, all on the BBC.

As an actor, he wanted to do as many different things as possible. He recalls his Yes Minister co-star Nigel Hawthorne saying to him one day, "Your biggest strength" - and I thought he was going to say his subtlety or something - "is your availability." Fowlds admits he used to love it when one job ended and there was what he calls "this wonderful insecurity" about not knowing what he'd be doing next. "I used to enjoy that in a funny way," he says.

"When I started Heartbeat I was 54, and when you're in your 50s or 60s to get a series that's successful and people love, you stay with it."

Basil Brush has recently been revived, without Fowlds or his creator Ivor Owen. The actor was prepared to criticise, but now realises there's a whole generation of kids who never saw the original and are impressed by the new Basil.

"I've only seen the new series on two or three occasions and it's not Basil. Basil will always be Ivor Owen," he adds.

Back in the 1970s it was a risk for an actor to share screen time with a glove puppet, even one as human as Basil Brush. "There were not many puppets on TV. I was a bit worried after the first series that I was a classically trained actor, so what was I doing with my arm round a bit of fur?" he admits.

"The second series took off for both of us. It was a rapport we came up with quite by accident. We did eight series and two Royal Command Performances. Ivor was a very special man. It was a lovely period."

He had a hard time after leaving Basil, retreating to the theatre for five years until Yes Minister came along in 1979. Of course, many of the political insights and scheming depicted are commonplace now, and you can understand when he says he wishes they were still doing it today. Both Hawthorne and Paul Eddington have passed away, and he says that Eddington's wife calls him the last musketeer.

"It was a great privilege to be with them and doing the show. Paul said to me they will be showing this forever. He always knew it was going to be a classic," says Fowlds.

He was 50 when he started Yes Minister, but says he never really grew up as an actor until Heartbeat "and time caught up with my face." His son, he adds, tells him he went from juvenile to geriatric with nothing in between. There's not much time for other things between series of Heartbeat, although he fitted in a documentary series for HTV as well as a few talking books and voiceovers. He has a home in Bath, which he rarely sees, and a flat in London, and rents a cottage in Yorkshire while filming Heartbeat.

"It's a different way of life and a different technique. Most of my generation of actors went to drama school to train for the theatre, and never thought about TV or film. It's all turned around," he says.

There are some interesting developments for Oscar towards the end of the next series. "But he's still an old curmudgeon in the bar and does the odd private detective work. Sadly romance doesn't come into his life," says Fowlds.

"Where else would I go? At 66 it's lovely to get up in the morning, play golf, have a swim, and go to work. I'm working with a fantastic crew, great cast and get paid at the same time.

"I know a lot of people knock Heartbeat but you can't take away the joy of the 11 million people who watch it. There's room for something like Heartbeat. I get a lot of letters from people saying 'thank goodness Heartbeat is there and you're still in it'. It's proved itself."

- Heartbeat returns to ITV1 on Sunday at 8pm