The dark recesses of the store rooms at Rattee and Kett in Cambridge reveal themselves like celestial prop departments. This is where bits of the House of Lords ceiling were soaked in resin before being regilded and stuck back on again; where for 23 years work has been carried out for Trinity College; where the new stonework to the designs of Quinlan Terry was cut for Downing College; and where Ely Cathedral has over the last nine years been pieced together again like an 11th-century stone construction kit. The last section is still being worked on.
Research is carried out, sometimes for years on end, before any stone is cut in a restoration project. Intricate drawings of Ely Cathedral litter the graphics department, 100 feet of one facade condensed on to a poster-sized sheet showing every brick, with the rotten ones scribbled over. The intensity of the pattern could be mistaken for an intricate piece of medieval needlepoint. Precise sizes, shades and other idosyncracies of each stone block are all noted and then imitated.
Labour intensive it may be, but even stonemasonry is touched by modern technology. The new mind-bending facility in the business is computer generated drawing, able to explode or shrink a design according to need, saving weeks in drawing time. For each cut of stone, a template is usually prepared in the graphics department first, which will then be used to guide cutting machines and masons through every angle of the design.
Only once all the drawings are complete is the stone prepared. Out in the yard, huge boulders are lowered into position on cutting tables beneath giant diamond-tipped circular saws, two-and-a-half metres in diameter. The blades are drenched with water as they are cut, washing out the heat and dust. 'Then the stone is dropped on to a Jenny Lind machine where it is polished,' says masonry supervisor Neil Fuller. 'The harder the stone, the greater the polish, so for instance marble comes up much better than limestone.'
All this takes place before the stone begins to take any sort of distinct shape. Then the lathe machines come into play, cutting shapes into columns, and planing machines to do the same on straight edges. A piece of the Henry VII chapel for Westminster Abbey zips backwards and forwards under a planing machine like the proverbial butter under a hot knife.
Then comes the skill of the human hand; the masons' eyebrows, hair and lips are covered in white dust as they knock their hammers and chisels on to the blocks, cutting arches, fleurs de lys and heraldic beasts along pencil lines drawn from the templates.
The stone carvers are set apart in a neighbouring workshop, where they create freehand. Here heraldic beasts emerge from the stone beneath their chisels. Moira Morris, a graduate in fine art, is copying a greyhound from a rippling clay figure she modelled first.
The tools are the same as those used by their medieval predecessors: splayed flat-ended pitching tools, another with a mushroom head to cushion the blows of the mallet. Only the tungsten inset of the chisel is modern, set with pinpricks of diamond to aid the cutting. There are modern air tools, too, the carver's equivalent of the dentist's drill. Tim Crawley, head carver, insists those who work with him use their hands as much as they can. 'If you use air tools all the time you get a distance from the stone. You need to get close to the stone and feel it.' He is an expert in medieval history but he is also gifted as a sculptor. Wrapped in an old sack in Crawley's workshop is a soft brown clay figure. It is one of a set which he hopes may be used to fill empty niches in Westminster Abbey. This particular statue, a woman draped in folds of cloth, has an extraordinary presence. If she finds her niche at Westminster, she may inspire all those who look at her for centuries to come.
Nimbus Conservation Ltd, Wadbury Barn, Mells, Frome, Somerset (0373 812545) has worked on Salisbury and Truro Cathedrals and Bath Abbey, mainly on decorative stonework.
Rattee and Kett, Purbeck Road, Cambridge CB2 2PG (0223 248061) has been restoring Ely Cathedral and Westminster Abbey, but will take on small domestic jobs.
J Layzell & Sons, Stoneleigh, Horton, Ilminster, Somerset (0460 52855) has just finished work on The Priest's House for the National Trust.
Peter Metcalf, 52 Hollings Street, Cottingley, Bingley, West Yorkshire (0274 569112) carries out restoration work and builds new stone houses.
M & G Stone Supplies, Walk Mills, Coney Lane, Keighley, Yorkshire BD21 5AR (0535 600528) specialises in reclaimed demolition materials and has a strong line in York paving, Welsh blue slates and the odd stone trough or lion.
'Cleaning Stone and Brick' by J Ashurst, a technical pamphlet from the Society for the Protection of Ancient Buildings, 37 Spital Square, London E1 6DY (071-377 1644). Price pounds 2 inc p&p.
'The Conservation of Building and Decorative Stone' by J Ashurst, F G Dimes and D B Honeyborne. Price pounds 95 plus pounds 2.50 p&p, Butterworth Heinemann, available from Reed Book Services Ltd, Northampton Rd, Rushden, Northamptonshire NN10 9RZ (0933 58521).
'English Heritage Practical Building Conservation Series Volume One: Stonemasonry' by J and N Ashurst. Price pounds 20.45 plus pounds 3.06 p&p. Available from English Heritage Postal Sales, PO Box 229, Northampton, NN6 9RY (0604 781163).