Smart people have told me my sitcom attempts should have a b-plot or even a c-plot.
Just pondering in the shower. Is this just an American thing? I can't remember Porridge or Fawlty Towers having a B-plot. Only Fools didn't until it became Hour long. I can't think Friday Night Dinner does either.

So, what do we think about b-plots?

I think you'll find they all have them, although it might be difficult to differentiate them if the sitcom's well written. Let's take an example off the top of my head. Fawlty Towers, The Germans.
Plot A: Sybil's having an operation, leaving Basil in charge.
Plot B: A moose's head has to be hung on the wall.
Plot C: the Major's trying to get a drink and wants to tell Basil some of his anecdotes.
And so on.

If they're done well, great. If they're shoehorned in, they suck. Take, for example, the finale to this year's run of Family Guy. It was a Brian and Stewie Road to ... episode. These episodes are usually great, but this one, Road to India, sucked the big one. Not least because it had a lame, utterly irrelevant B-plot involving Peter playing bingo and being a boring, unpleasant idiot.

South Park creator Trey Parker talks about what a great epiphany it was for him when he realized that episodes didn't require a B-plot, so he could focus more on having fun with the story.

A B-plot works best when it is an integral and seamless part of the main story. For example, in Series 7 of Minder, there's a superb episode called The Last Video Show. The main plot is that a big-time gangster boss (Ian McShane) has a compromising sex tape of a corrupt police detective (beautifully played by a beardless Brian Blessed), but the tape ends up at Arthur Daley's video shop -- and hilarity and punches ensue as everyone tries to get their hands on it. The B-plot is that Arthur is selling nicked residential fittings (stolen by dim mechanic Arnie [Ray Winstone] and his "echo"). No connection to the sex-tape, but toward the end, these two plots merge perfectly. You can view the Minder episode on YouTube: https://www.youtube.com/watch?v=qJHrhnunNOk And of course, we should all buy it on DVD.