When Oliver Twist was introduced to the art of pickpocketing, one of the first “customers” he encountered was a gentleman riffling through books on a stall in Clerkenwell Close, whose nearby alleyways and rookeries were home to some of London’s most infamous villains, and an area Charles Dickens had lived in and known well.

Fagin, Oliver’s professor in crime, was based on a real thief and fence who spent time in a prison just around the corner: a frightening, lightless enchainment of cells, underground passages and ventilation tunnels in which, until the Victorians reformed it on the lines of the new Pentonville Gaol, men, women and children were incarcerated higgledy-piggledy awaiting trial for offences ranging from handkerchief-stealing to horrific murders.

The place was eventually sealed up and a school erected over it, and it was only recently that The House of Detention, as it became known in 1847, was opened to the public. You can now witness in sight, sound and atmosphere all the horrors of 17th and 18th-century prison conditions, the period of convict transportation and the Victorian Era of Penal Reform, a fascinating, shivery dip into history which will not easily be forgotten.

Escorted by your guide, a young man steeped in those times, you grope along dark damp corridors, twist and turn past cells and dungeons, discover laundry rooms where women scrub and mangle, silently, backbreakingly and see where the filthiest rags were fumigated or burned in an adjoining chamber.

It took two cooks three-and-a-half hours to weigh out the food rations for this enormous institution housing more than 10,000 in a year, and as you approach the kitchens, a door may open revealing, in the half-dark, a figure, apron loaded with . . . what? Might it be potatoes? More likely, bullocks’ heads for the staple diet of soup and boiled meat with yellow fat.

In pre-Victorian times convicts were masked to prevent them socialising. Money for food was not only extorted from the inmates but from those who had been granted their freedom, otherwise many were left there to starve. You can enter cells intended for those who had committed social crimes (prostitutes) and petty crimes, or were placed in penal servitude, and learn about the notorious Jack Shepherd, who escaped with his girlfriend by sawing through his leg irons in 1724. You will pass figures standing in stone water-tanks in the bitter cold, washing in buckets, and see some brutal surgical instruments. Yet nothing is sensationalised. It’s a picture of prison life as it was, with no holds barred.

For a bit of fresh air to blow away that dismal experience it’s worth making your way around the passages, narrow streets, hidden gardens and modern offices which encircle Clerkenwell Green, and through the magnificent Smithfield Market as far as King Edward Street, to the pocket-sized Postmen’s Park.

Here the postmen from the old main post office would take a break under the trees. At one end you will find a shelter with benches. This is framed by 32 ceramic plaques, charmingly lettered by the artist G F Watts, who proposed in 1887 an idea to erect a series of memorials to ordinary local folk who died saving the lives of others.

Many were children: “John Clinton aged 10 who was drowned near London Bridge in trying to save a companion younger than himself July 1894” reads one, and another: “Soloman Galaman aged l0 died of injuries Sep 1901 after saving his little brother from being run over in Commercial Street died in hospital saying: ‘Mother I saved him but I could not save myself”.”

Henry James Bristow, aged eight, from Walthamstow, was left alone with his three-year-old sister when the toddler upset a paraffin lamp and the boy tore off her flaming clothes, but caught fire himself. He died of burns and shock on December 30, 1890.

The entire outing makes a thought-provoking encounter with the Victorians which taps into two very different facets of their times.

* The House of Detention, Clerkenwell Close, Clerkenwell, London EC1R 0AS. Groups book in advance. Teacher packs available. Tel: 0171 253 9494 * The Postmen’s Park, King Edward Street, London ECl