The Other Darrin: In "Newcastle", Benedict Cumberbatch was ill with Laryngitis. As there was no time to re-record the episode when he was well, Tom Goodman-Hill filled in as Martin. Although he didn't sound exactly the same, he did an excellent job of mimicking Cumberbatch's speech patterns. This is heavily lampshaded in the pre-credits opening scene.
Arthur: Mum, we've only got two pilots. What would we do if one of them went sick?
Carolyn: They wouldn't dare.
Arthur: But what if they did?
Carolyn: Well, I'm sure we'd think of something.
[Door opens and Martin enters - played by Tom Goodman-Hill]
Carolyn: Ah, there you are, Martin.
Martin: Hello Carolyn, what have I missed?
Arthur: Hi Skip, you're looking very well.
Martin: Oh, thank you Arthur.
Arthur: Don't you think, Douglas?
Douglas: Not especially. I think he looks exactly the same as always.