When Sherlock creators Mark Gatiss and Steven Moffat revealed their now traditional ‘three key words’ ahead of the new series, there was one in particular that filled fans with excitement – Sherrinford.

That was the name Sir Arthur Conan Doyle had originally been considering for his great detective before finally settling on Sherlock. While it never appears in Doyle's canon, it’s the name proposed for a third Holmes brother by noted Sherlock Holmes scholar William S Baring-Gould. Baring-Gould notes that Holmes revealed in one of Doyle's stories that his family were country squires and reasons that, since the eldest brother of such families traditionally stayed at home to manage the estate, Holmes and his brother Mycroft – a detective and a senior civil servant, respectively – must have had a third brother.

When Mycroft (played by Gatiss) told the detective at the end of series three of Sherlock “I’m not given to outbursts of brotherly compassion. You know what happened to the other one…” fans latched on to it, not only hoping to see a third Holmes brother in series four, but also assuming that if they did, his name would be Sherrinford (and, in many cases, that he would be played by Tom Hiddleston).

So when Mycroft made a phonecall towards the end of series four opener The Six Thatchers, asking the person on the other end of the line “Put me through to Sherrinford, please,” you can only imagine what the reaction must have been in certain living rooms around the country (and the world).

However, episode two, The Lying Detective, threw a potential spanner in the works. The third Holmes brother is actually a Holmes sister, Euros, which – unless Holmes siblings are going to keep coming out of the woodwork – means Sherrinford must be someone or something else entirely. But who, or what?

So far, the language around Sherrinford has been deliberately ambiguous. “Put me through to Sherrinford, please," says Mycroft on the phone in episode one. Could be a person, could be an organisation. "Do you still speak to Sherrinford?" Lady Smallwood asks him in episode two. "I get regular updates," says Mycroft. "And?" "Sherrinford is secure." Sounds like a person but, again, could be something else.

Whoever or whatever Sherrinford is, it's clear he, she or it is something pretty weighty. The gravity in Mycroft and Lady Smallwood's conversation is clear and anything that has to be checked up on and kept secure sounds potentially dangerous.

It also sounds like it might be slightly out of Mycroft's control – which must be an unusual feeling for someone who appears to be pretty much the biggest player in British government...Who or what is Sherrinford in Sherlock?