Lieutenant Colonel Kevin Roberts is too diplomatic to name his trickiest assignment. The senior director of music for the Household Division, the British army regiments that serve as the Queen’s official bodyguards, Lt Col Roberts is the man responsible for ensuring that one of the most important parts of a state visit goes off without a hitch: the playing of the respective national anthems.

The slightest slip can cause offence and sour an important meeting of heads of state. To guard against such a potentially tricky outcome, Lt Col Roberts researches the anthem as soon as he is told about a forthcoming state visit. In many cases, countries will provide an official recording played by a national band or state orchestra. After that he will book an audition with the relevant embassy. “Sometimes they’ll send us the sheet music because anthems can change overnight or with a new government, and we have to be very careful that we play the right tune.”

The Household Division has one mounted band and five from the Foot Guards. Each band takes part in several hundred engagements every year, which can involve as few members as a single bugler, through a quintet – at a state banquet, for example – and up to the full, 200-strong massed band used for large state ceremonies. “We have played in excess of 40 different national anthems and our total performances of national anthems number in the hundreds,” says Lt Col Roberts.

The Household Division bands do not have the luxury of improvising and they cannot transpose the key of a piece to make it easier for a military band to play. “We are the custodians of getting it right,” he says, noting the wide scope of musical styles employed by nations to project their heritage and self-image. As a benchmark, he picks an anthem he knows very well indeed. “‘God Save the Queen’ is simplistic in form: block chords and easy to sing. But some anthems are 10 times as long as that and have mini overtures or little tiny operas [within them] four minutes long.”

The challenge of playing a complex national anthem in front of an audience that will spot the tiniest bum note is demanding enough. But the Household Division must also perform its repertoire while marching in time, wearing hearing protection and balancing 45cm-tall, 680g visibility-restricting bearskin hats on their heads.

And unlike their pampered orchestral colleagues, they have to play in all weather. “We were preparing to play on a frozen lake in Switzerland when the brass instruments completely froze,” says Lt Col Roberts. “Trombonists couldn’t move their slides, no one could move valves. As the VIP arrived the band were frantically heavy-breathing on their instruments. Luckily we got away with it.”