Maybe not quite that far, the guardroom (around 1963-68 time) was more or less in line with the Chapel. To the left of the barrier (actually by the road before you reached the barrier) was the Adjustant's office (became the snack bar); the guardroom was just beyond the barrier in front of the Chapel (i.e. to the left of it as viewed from the corner) and first bit of the school. David has it wrong here, the guardroom has gone, building on left was the Adjustant / later snack bar.