We had to find out more, so we gave Martin a ring to chat about his character, twists and turns, why he's a rubbish blagger, and snogging Sheila Hancock...
Can you give us a summary of what your episode of Hustle is about?
"I play a character called Harry Holmes, who is kind of a gangster. He's in possession of a Picasso, a stolen Picasso. So I think the grifters think that's kind of fair game, really, and they come after the stolen Picasso."
What's Harry like, apart from being a gangster?
"If you've seen Hustle before, Hustle is kind of like one degree over reality. It's always heightened, which makes it fun to watch in the way that they play it, in the way that stories are, even the way that it's shot - the camerawork is heightened and it's meant to do that.
"So the character of Harry Holmes is exactly that - it's kind of heightened realism gangster. I think I've taken a bit of the Kray twins movie that I did, I've taken a bit of Steve Owen [from EastEnders]. For me, it was kind of like stepping back into comfortable shoes, playing the gangster again and having a bit of fun with it."
Did you enjoy going a bit over the top?
"Yeah, absolutely - it is fun. That's what I mean, it was a comfortable pair of shoes but it was fun lifting it above reality. I enjoyed it."
What is it about bad guys that you like playing?
[laughs] "What is it? It's because I never get offered to play the good guy! But listen, it is fun. It's fun being on a film set full stop. It's fun for me playing bad guy or good guy. I love being on a film set. I think if I couldn't get a job I'd be a runner, because I love being there - it is just a pleasure."
This is quite a starry episode - did you get to work with your fellow guest stars Sheila Hancock or Bill Bailey?
"Sheila, yeah. It's quite funny, because the last time I worked with Sheila we were snogging in EastEnders and this time I get a kiss from her as well, so there you go!"
Did it remind you of old times?
"Yeah, it did!"
This episode was directed by cast member Adrian Lester - what was it like working with him?
"That's right. It was really nice. It took the pressure off me. It's quite hard sometimes coming into a show where everybody's quite cliquey, where everybody's been set for a number of years and you're the guest star. It's quite difficult, it's nervewracking in a way. But I think Adrian directing for the first time took the pressure off, which was really nice."
Was he a good director?
"He's really good. I only recently directed my first feature, so we spoke about it a lot and the difficulty of doing it for the first time. I think because I had that experience just recently I understood where he was coming from a lot more."
We spoke to stars Kelly Adams and Matt Di Angelo recently and they told us this is a very complicated episode...
"It is complicated. That's why I'm keeping it very simple!"
Did you understand it?
"Do you know what? It's best sometimes, when things are complicated, if you just understand your own part. Because if you start delving into all of it, then you can come up with so many coincidences and too many things that are right and wrong with it.
"So it's best sometimes when you're going in and doing a guest spot where your story doesn't leak over into the others if you strip down your script so you don't even look at it. You just look at your storyline and your plot and you make sure you get that right without wondering about everybody else's."
It seems like one of those episodes where you don't know who to trust.
"Oh absolutely, yeah. There's several suspects and several villains and you don't know who to trust, but you know definitely not to trust me! That's always the case!"
Can we expect some twists and turns?
"Oh yeah, there always is. In those shows - Hustle, Spooks - there's always twists and turns. That's how they're built."
Do you have a favourite part from the episode? Apart from kissing Sheila, obviously.
"Not really! I enjoyed all of it, I have to say. I just enjoyed being on the set and working with Adrian. He was really nice. And meeting the guys."
How good are you at blagging?
"You know what? I'm not very good. I think I'm very good at being honest, and that's kind of how I've grown up. As a kid, I was always the honest kid. I think the way my mum and dad brought me up was that you have respect for yourself, but more than that you have respect for other people and other people's things.
"I think with that kind of in-built in me, I'm not very good at it at all. If we go to a market and [my wife] Shirley's trying to buy some kind of vintage lace or something and she's haggling over it, I'd much rather say, 'Look, just give her the tenner!'"
Have you got any other acting projects coming up?
"Not at the moment that I can mention. There are a couple of other things, but not that I can talk about yet!"
If someone asked you why they should watch your episode of Hustle, what would you tell them?
"Just enjoy it! It's fun. It's a lot of fun, and it's just good drama."
Martin Kemp guest stars in the new series of Hustle on Friday, January 20 at 9pm on BBC One.