Julian Cripps, our guide, began with a question: 'How long do you think it took to build?' He then confided that some people answer 'two years', others 'a lifetime'. Two years, I thought. You do get some pretty dense people on a guided tour! But my own guess wasn't a lot better. Perhaps a century, I thought, but our guide responded with 'No way - this place took 252 years to put up. It was started in 1220 and wasnt finished until 1472'.

The building that took twelve generations to complete then began collapsing almost straight away. Made of magnesian limestone, it has been crumbling for the last few hundred years, and only in 1999 was the scaffolding taken down to reveal the results of the most recent phase of renovation work.

'Magnesian limestone is a beautiful material,' Julian enthuses. 'Just look at the colour on it. But it is also very soft. It makes it wonderful for carving, but the down side is that it simply crumbles away. The industrial revolution came along, and then the advent of the motor car and modern pollution, and the cathedral has been crumbling ever since.'

The limestone was quarried in Tadcaster, 9 miles (14km) southwest of York, from a quarry that still exists today. The materials were brought into York and gave their name to Stonegate, the street that leads up to the cathedral.

Julian points out a modern frieze around a door, where the original had to be completely replaced. It tells the story of Creation and was designed by Rory Young, a young sculptor from Cirencester. The detail allowed by the delicate limestone is impressive, though not to everyone. 'A woman on one of my tours,' says Julian, 'said that can't be Eve because Eve, being born of God, did not have a belly-button. I told her she'd better take it up with the sculptor.'

Once inside, we soon realise that a tour with an informed guide brings the cathedral alive in a way that no guidebook could possibly do. Julian points out that York Minster was deliberately built to be twenty feet wider than Canterbury Cathedral, because of the fierce rivalry that has always existed between the two main religious seats in England. 'Both Archbishops claimed to be head of the church in England,' Julian tells me, 'But the dispute that was settled by a fine compromise in about 1700 when York was declared the Archbishop of England and Canterbury the Archbishop of All England'.

As he points out the Archbishop's Chair, Julian surprises us by saying that the Archbishop of York has no authority in York Minster. He cannot even visit his own cathedral without an invitation from the Dean.

With great enthusiasm, Julian shows us hidden gems, the obvious grandeur and even the errors made in the building of the cathedral. An archway in the south aisle is clearly out-of-true, although you would probably not notice at first sight. 'They didn't have plumb-lines and spirit levels in those days,' Julian explains, 'so it was mostly a case of the rule of thumb and lining things up by sight. The whole cathedral leans by about six inches from the perpendicular.'

Julian shows us the tomb of an archbishop who has been given two right feet by the sculptor, and as we exit through the south door we pause a while at another tomb. 'This is the tomb of Walter de Gray,' Julian tells us, 'who was archbishop from 1215-55 and is the man who had the vision to begin the construction of this building.'

Perhaps some of his workmen didn't quite have the same vision, because as we leave Julian points out one last anomaly in the building. 'The nave,' he says, 'isn't level. It's on a slope. If you were to put a marble down on the floor of the cathedral it would roll away.' Lacking a marble we take Julian's word for it, and exit both enlightened and entertained.