FORTY YOUNGSTERS wearing green overalls, numbered tunics and serious expressions emerge from the building and march single file across the square.
"They're looking a bit glum," explains Lt Col Charles Simmons. "They have just spent an hour going through a written planning exercise." He hands me a copy of one group's test and the reason for the serious faces soon becomes apparent.
They are in a lakeside village in Kenya where the menfolk have left on a fishing expedition. News comes in that bandits are advancing on the village, having left wounded in their wake.
No other community is less than 100 miles away and it takes five hours to cross the lake . . . and so on. There are distances to memorise and types of transport which travel at different speeds.
After an hour going over this, the candidates go off into their groups of eight. They have 15 minutes in a classroom to discuss it as a group, then they stand up and are grilled about their plans to save the villagers and wounded.
I am watching Red Group. There is quite a mix here - two of them serving soldiers, four graduates, and two school-leavers. Two of the group are women.
Group leader Lt Col Jac Bazzard sits in a corner taking notes. "OK number eight," he says sternly. "You have 10 seconds to sum up your plan."
Welcome to the Regular Commissions Board, the Army's testing ground for potential officers.
Here at Leighton House, a former country home in Westbury, Wiltshire, some 1,300 young candidates go through the board's rigorous testing each year in their quest to enter Sandhurst and on to a career as an Army officer.
The candidates will already have visited the centre for a briefing, when they are given a taste of what's to come and those considered unsuitable are weeded out.
But now it's crunch time. Over three and a half days they take written tests on general knowledge, service knowledge and current affairs. They will be asked to discuss topics in a group, and will be interviewed individually.
There are also outdoor tasks designed to put them under stress, make them think on their feet and see how they react in a group. In short, it's pretty tough.
The methods of the Commissions Board go back to the Forties - ironically, they were first adopted in the German army. Psychologists play a huge part in the testing. Even the assault course was devised by academics.
Out in the grounds, on what at first sight resembles a giant adventure playground, Red Group is now being put through its paces on command tasks.
On one task they have three planks, a pole and some rope and have to get themselves and a heavy box across three high platforms yards apart without touching the ground. As they battle across it, Lt Col Stewart Walker points out the candidate who has been picked to lead on this task.
"This guy is allowing events to overtake him, which is a shame," he murmurs. "We are getting a general impression - nice guy, good manner, but he's appearing to lose it."
Lt Col Walker says an assessor's job can be hard work over the three and a half days. "We must appear to be a stony-faced lot. But we can't be seen to smile at the candidates or laugh if one cracks a joke because it would give him more encouragement."
The general impression the board exudes is that they're not out to break your spirit. They want to encourage would-be recruits to get through, but only if they're made of the "right stuff". The pass rate for male applicants is 61 per cent, and for women, 67 per cent, although fewer females apply.
The board is keen to open its doors to schools and universities. Among those observing the RCB tests is Barnaby Lenon, headmaster of Trinity School in Croydon, south London.
He says: "I think it's very impressive. I wish I had three days to assess all the people who apply to my school. Ours is an independent boys' school with 900 pupils and we are going to have a small number interested in the armed forces.
"I am here for two reasons. One, so I can better advise my pupils who are thinking of applying to become Army officers. And two, because I'm interested in the whole process of staff selection."
The process has also left its mark on the candidates themselves. "When you look back, it has been good fun," says Joe Dransfield, a 22-year-old graduate from Exeter who is hoping to become an officer in the Royal Engineers.
"I have always been keen and had contact with the services since I was about 15. I chose my GCSEs with a services career in mind."
In the evening the candidates will be treated to dinner in the officers' mess while the assessors compile their results.
The next morning, after the last test, a team run across the assault course, the final "boarding" takes place. Candidates are discussed in turn and scored between nought and nine over twelve categories, which include intelligence, educational standard, communication, analysis and planning, problem-solving and reaction to stress.
For two hours each member of Red Group is discussed and scores are given using this points system. The assessors also take into account reports from headmasters and college tutors, which can be vital in borderline cases.
Finally, out of the eight candidates, five have passed, one with a merit. Two of the passes have scraped through - they will have to take further courses before they can go on to Sandhurst.