Heavenly is perhaps the most appropriate word I can think of to describe the adventure I have just been privileged to experience.

Mind you, to embark on it I had to demonstrate I'm reasonably fit, have a head for heights, can climb up and down 250 steps in various narrow and unlit spiral staircases...and can just about squeeze through an archway a mere 14 inches wide.

Having proved that I do indeed possess those qualifications, and signed the obligatory 'elf'n'safety form, the expedition I was able to join was in Wells Cathedral.

Known as the High Parts Tour, it is as thrilling and breathtaking as an adventure park ride as it takes you to elevated areas of the ancient building which are only accessible to a couple of hundred lucky visitors every year.

The tour gives a chance to explore all sorts of nooks, crannies and unexpected galleries which run through the walls high about the nave and behind that famous line-up of stone statues on the West Front, then up to the roof of the great central tower for panoramic views of the Somerset countryside.

On the way, the guides point out marks inscribed by the masons working on the stonework 800 years ago, the secret joints they made on the ancient oak beams along the triforium, the hidden flying buttresses, and the graffiti left – in some cases hundreds of years ago – by generations of choristers, schoolchildren and visitors wanting to prove they wuz here. Those who shepherd you through cobwebs in the dusty corridors talk of the way the original designers and stone carvers marked out their ideas in thin plaster on the floor of a lofty drawing room to help the work of their colleagues hacking rock from nearby quarries.

They describe how 14th and 15th-century processions across the green space in front of the cathedral were greeted by seemingly angelic voices and trumpet notes coming as if by magic from chiselled features of the saints and bishops in their high niches on the West Front...all thanks to choristers and musicians using the medieval version of megaphones carved into the fabric of the thick walls.

They show the exact points in the masonry where new methods brought spectacular changes, and they describe the many extensions and additions that have been made through the years, including rebuilding the central tower to its present 182ft height but making it so heavy that the cathedral's spectacular inverted "scissors" arches had to be created to stop it from collapsing.

Finally, a chance of a behind-the-scenes view of all the cogs, springs, pulleys and eccentric lengths of wire which operate the unique 14th-century astronomical clock in the north transept, cue the medieval knights to joust every 15 minutes around the top and remind the venerable figure of Jack Blandiver to bang away at the bells beside his seat with his heels so that no one can have the least doubt about the time.

All in all, a memorable tour – made even more so by the glorious sound of the International Children's Choir, from America, rehearsing in the nave for a concert later in the day.

The High Parts Tour is available only to organised groups of up to 12 people. For details, contact visits coordinator Ruth Clacee-Rowe in the Cathedral office on 01749 674483 or email her at visits@wellscathedral.uk.net

Ian Tabrett