Played with in the Series 3 episode 'Paris'. According to John Finnemore's blog, he promised Benedict that he wouldn't put in Sherlock references just for the sake of it. True to his word, in an episode where Martin attempts to track down a missing bottle of whisky with his trusty sidekick Arthur, he is repeatedly likened to... Miss Marple. He also narrowly avoids quoting Sherlock Holmes in the line;
Martin: The thing is, we've taken away all the things that can't possibly have happened, so I suppose the only thing that's left, even though it seems really weird, must be the thing that did happen in fact!
Douglas: ...Snappily put.
John Finnemore's promise to Benedict didn't stop him from having Martin's ring eaten by a goose in the season four episode 'Uskerty'. He thought fans wouldn't notice the reference to The Blue Carbuncle. He was very, very wrong.
It didn't stop Martin's brother Simon from being a minor government official in "Wokingham" either.
In 'Limerick', Douglas reminds an over dramatic Martin that he's not in an Arthur Miller play. He'd know.
Hey, It's That Voice!: Besides the main cast, there are rather a lot of guest stars, including recurring character Hercules Shipwright, played by Anthony Head of Buffy fame (who gets in a warm fuzzy voice contest with Douglas).