Novel problem: Texan trapped in Waterstones bookstore

Having the free run of a bookstore might be the stuff of some people's dreams, but for one tourist in London it turned into something of a nightmare.

David Willis, from Dallas, Texas, was accidentally locked inside a Waterstones store in Trafalgar Square, London, on Thursday evening. Waterstones is Britain's largest bookstore chain by sales, according to industry research firm IBISWorld, with 37 percent of the market share. It has close to 300 stores across the country.

Willis sent his first tweet about his predicament – along with an Instagram image of closed shutters – at 10.11 p.m., claiming he had been upstairs in the store for "15 minutes" and "came down to all the lights out and door locked".

A call for help came next, at10.55 p.m. BST, and was retweeted over 8,000 times.

Twitter users rallied behind the trapped man, many using the hashtag #WaterstonesTexan and #FreeTheWaterstones1.

But although Willis' plight might have caught the attention of thousands on Twitter, it was some time before his pleas were heard by Waterstones' social media team, who appeared to have clocked off for the night some hours before.

In the end, it was London's Metropolitan Police that came to Willis' rescue at 11.00 p.m.

"We were called at about 9.35 p.m. by a man who claimed to be locked inside a shop. We attended, and the man left the building at around 11 p.m. after the arrival of a key holder," a spokesperson for the Met Police told CNBC on Friday morning

Willis was freed, Twitter users celebrated and Twitter's data team posted a graph of the total number of tweets about the saga (9,704).

- By CNBC's Katrina Bishop