You’re looking very dapper. Is it a relief to be out of the chainmail?

Let’s just say it’s nice to take it off. I wear chainmail day in, day out. Chainmail amplifies your temperature so if it’s hot, you cook, and if it’s cold, you freeze. I wear it for 12 hours a day and by the end…well, you know about it!

What does this series hold for Arthur?

Three years have passed between this series and the last because Camelot has been enjoying prosperity and peace, which isn’t interesting from a dramatic perspective. So Arthur has now been king for four years and knows what he’s doing. But evil is about to rear its ugly head again…

Can you reveal any of the guest stars?

Gary Lewis, who plays Alator, is coming back. In series four he was a red herring: you thought he was bad but he turned out to be good. I was so pleased the first time he turned up on set because I think he’s a fantastic actor.

And we’ve heard a rumour that another familiar face is making a comeback?

Anthony Head? He does. Fans will recall his character died last year so he’s looking a bit paler.

What skills have you acquired in five series of Merlin?

Horse riding, sword-fighting, how to sign my name at the speed of light. When asked for my autograph, I used to take a lot of time over the signature and write a little message. Now I get out of there as quickly as possible! Otherwise they smell fresh mean and suddenly come at you from all sides. It’s like that zombie film, 28 Days Later.

And what are you still struggling with?

French. We film much of it at a chateau in northern France and after all these years you’d think I’d be fluent. I can just about order a croissant and most things, although I don’t always know what I’m ordering.

Are you inundated with fan mail?

You get an idea of when the show is airing in specific parts of the world – a lot of post from Italy one month and Australia the next. It goes in waves. A lot of people have been able to relate to the show, no matter what country they come from: that’s really flattering.

What’s the average age of your fans?

Something like 8% of the audience is under 18, which surprised me. It’s a very wide range. As the years have gone by, they’ve steered it away from being a kids’ show. It’s grown-up. We’ve grown-up.

Can you believe Merlin’s success?

I can. I don’t want to sound arrogant but from the first series what I picked up on was the enthusiasm of the people behind the show. It made me believe that it would be something. So I’ve never been that surprised by its success.

If Merlin goes to a tenth series, will you?

I think there will be a time to throw in the towel. When the drive isn’t there any more, that’s when it’s time to stop. They’re in talks about series six at the moment but I think we’re going to see how this series goes before thinking about series six or anything. That’s been the philosophy at the end of each year. The key is you don’t want to do something that isn’t better than what we’ve just done.

What would you do?

I imagine bar work. Maybe a paper round. No, I’m really excited about the future because I’ve had a great platform and whatever it brings I’ll be able to look back on Merlin with some really fond memories and having learnt a hell of a lot that I wouldn’t have been able to do on anything else. 

Series five of Merlin starts on Saturday at 7:45pm on BBC1