I've seen bits in...more than one Sherlock story to the effect that John has the wrong build for rugby, so I thought I'd say...no, he doesn't. Really! I know a lot of people think it's all huge people hitting each other, but it's not.

I'm an ex-rugby player, and if a man John's size and shape told me he'd played, my reaction would assuredly not be "pull the other one". John (well, Martin Freeman) is 5'7". That's Peter Stringer's height, and Stringer played for Ireland! There are a number of pro players who come in at 5'5" or 5'6".

The smallest male rugby player I've ever met is about 5'4" and probably around 150 lbs. He played hooker, and was extremely small for that position, but his quick foot was much more important than his lack of size.

Point is, there's not a particular build that works or doesn't work for rugby, and Sherlock's version of John Watson is well within the (very large) range of builds that do.

Also! If you are a Sherlock writer who is putting some rugby bits into your story, feel free to ask me to take a look. I played rugby union, both pre- and post- the 2007 rules changes.