Nigel Farage, the Trump-May go-between?

Nigel Farage helped Donald Trump campaign

There will be "no third person" in the relationship between Theresa May and Donald Trump, Downing Street has opined, intentionally or not, echoing that famous phrase of Princess Diana's describing the difficulties in her marriage.

Nigel Farage clearly is hoping to be a giant gooseberry in the relationship between the prime minister and the president-elect. But can he take a role? Should he? And does his grinning picture outside the Trump Towers' golden lift actually create any problems for Mrs May?

As the PM prepares to give her first big foreign policy speech at the Guildhall, Mr Farage's antics are clearly deeply annoying for Number 10. One source said they were "maddening".

Mrs May's team had no prior knowledge of the meeting, and found out the same way as everybody else, when Mr Farage turned up at Trump Towers in New York, before posting a picture with a smile as wide as Mr Trump's ego after their meeting, and later apparently enjoying a cigarette break on the balcony of the his penthouse.

Just at the moment, when Mrs May's government is trying to get to grips with a new administration - a largely unexpected one - across the pond, Mr Farage pulls off one of his biggest achievements in a career of winding up Conservatives.

And he's already visited the president-elect in New York

But let's be clear. Despite a few MPs suggesting the contrary, it is about as likely as Mrs May taking selfies with 10-year-old Barron Trump on the patio outside the Cabinet Room that she will end up giving Mr Farage an official role.

But right at the time when Number 10 is trying to organise its visit to Washington DC, after being the ninth seat of government to receive a call from Trump Towers, Mr Farage's mini-break certainly looks bad for Mrs May, implying that he has more clout than she does, that it's the UKIP leader, not the prime minister, in charge of our ties with the US.

It matters not, perhaps, because it was Mr Farage, or because of the temporary story about the embarrassment. But because the relationship between the UK and the US matters enormously, and like any diplomatic tie, it is created and maintained through the painstaking application of protocol.

The Farage-Trump visit demonstrates, as one diplomat put it, that Mr Trump is "deeply unorthodox", that he is not interested in sticking to any convention.

That was, after all, a huge part of his political appeal for those who backed him. But for Mrs May, as she embarks on building a relationship with Trump and his team, there's no predicting what he will do.

And she has also been criticised from the left, for refusing to condemn some of his most controversial beliefs. The prime minister finds herself with no map, little guide on how her most important ally will behave.

Whether or not Mr Farage keeps popping up, that's a problem she could do without.