IN AN unremarkable corner of Westminster Abbey is a wooden door marked "Private". Behind it are 78 wooden steps spiralling upwards in a narrow staircase. And at the top of those is one of the Abbey's hidden treasures: what John Betjeman once called "the greatest view in Europe".

More than 20 metres above the floor of the Abbey, and largely invisible to the tourists taking pictures below, is a vaulted gallery that runs the entire length of the building. This is the Abbey's eastern triforium, a centuries-old secret expanse that is to be opened to visitors for the first time as part of a gallery and exhibition space.

The Dean, Dr John Hall, invited us, a group of reporters, to join him in a final tour around the triforium before building work begins in earnest to transform the dusty space into the Queen's Diamond Jubilee Galleries (News, 6 March).

The gallery is surprisingly spacious, and is well lit by large windows that offer views across to the Houses of Parliament. Dr Hall explained that the triforium, which is larger than that in most cathedrals, was designed to be used as chapels for the monks, but was never actually furnished. Purbeck marble columns and stone gargoyles carved into the ceilings indicate that the space, while hidden from view below, was always intended to be used. Huge oaken beams criss-cross the galleries, put in by Sir Christopher Wren to support the roof.

Squat brown pipes running alongside our feet are evidence of more-recent architectural interventions, and provide heating for the whole building. When the triforium is re-opened as a museum and gallery, the pipes will be sunk beneath a new floor.

Rather than tackling the daunting spiral staircase, visitors will be whisked up by a lift in a newly built tower outside the Abbey's walls, and will then step across a walkway into the refurbished triforium.

It is impossible to spend much time in the space without shuffling towards the edge of the gallery to take in the views across the Abbey, which are impressive. At one end, you can see down the entire length of the building, where tiny tourists scurry across the floors. It was this sight, previously restricted to Abbey staff or Richard Dimbleby commentating on the 1953 coronation service for the BBC, that Betjeman called the finest in Europe.

As Dr Hall led us through the triforium, men and women were busily working around us - packaging old masonry and artwork, long stored in the galleries; vacuuming the dusty space in preparation for the builders. Under a spotlight, a restorer worked patiently at piecing together a chipped and fragmented plaster cast of the funeral effigy of Margaret Beaufort, grandmother of Henry VIII, who is buried elsewhere in the Abbey.

It was impossible to miss the many names carved into the old stone walls. A mildly embarrassed Dr Hall explained that these were probably the handiwork of pupils from Westminster School, a prestigious private school located in the Abbey precincts, who were prone to sneak into forbidden places and leave their names behind.

The Queen's Diamond Jubilee Galleries, the curator, Susan Jenkins, says, will hugely increase the number of ancient treasures that the Abbey is able to show to the public. The Galleries are expected to cost £19 million, of which £11 million has already been raised, and are to open in 2018.