Some of his tools, too, are more often seen in museums. For certain types of flint knapping and for fine finishing, Bernard uses pieces of deer antler, which jostle with old coal-breaking hammers in his plastic bucket. His wheelbarrow is full of 16th-century lime mortar, which he has been crushing so that he can re- use it on the walls he restores.
'Lovely stuff,' he says. 'Modern cement mortar is terrible. It has no suction with the flint at all. I have just been to see a flint building at St Albans which is ruined because the flints don't like cement mortar. You don't use a steel hammer for neolithic reproduction work. It takes off a different sort of flake. The steel makes a very deep shave.'
To demonstrate, he turns the mottled blue-white knob of a flint over in his hand and cracks it with a steel hammer. A sliver of flint splits away revealing the almost black core, slightly swollen like the shape of a mussel shell. 'That is called the bulb of percussion, and you get that with a steel hammer,' he says. 'With deer antler you get a flat cut.'
The discarded slivers might be used as 'gallets', little stoppers in a wall between larger knapped flints, designed to give the darkness of the wall greater density. He is making safe the ruins of a church at Roudham in Norfolk, so that villagers can continue to hold occasional services among the roosting pigeons.
The church walls are random flush work, which means that each cut flint is placed haphazardly with its flat black core facing outwards. There are chips and dimples in the flints, showing where the hammer has worked. Were this 'fine' flush work, each one would be smooth as marble.
The skill of flint knapping lies partly in being able to 'read' a flint. You or I might hit a flint for hours and never achieve a break. The core inside the white, crusty outer cortex is made of silica, which has no grain at all and is only slightly softer than diamond.
The experienced knapper will first look for the angles before he cuts it with a single strike. 'I look for a certain kind of platform,' Bernard says. 'It is no good striking a flat surface. It can get very technical explaining how different angles respond, but basically it's the angles you go for.
'Some people say you can turn the flint round in your hand, tapping it all the way, and you get a clean break, but it doesn't work. That way you just send shock waves through it.'
Flint that has been freshly dug has a slight moisture content and is easier to work than flint that has been exposed for many years. Those that have frost damage simply disintegrate when they are cut. You can tell a damaged flint by tapping it: a good one rings like a bell; a bad one sounds muffled. 'Most flints have fossils inside them because they were originally formed round fossils, usually sponge fossils,' Bernard explains.
The finest square flints will take at least 20 minutes each to cut and prepare, while a rough-cut flint will take around five minutes - 'But some houses actually have very few flints. Some of those along the coast that I've worked on have only around 20 flints per square foot.' For a suitably weathered look to new flint, a solution of fresh cow dung can be painted on to encourage the growth of moss and lichen.
Flint as a building material lasts for thousands of years, provided that water doesn't leak in through the mortar - something much more likely to happen with modern cement mortar than with flexible lime mortar. Bernard, one of few craftsmen with the knowledge and skill to work with flints in this country, believes a revival is under way: 'A lot of younger architects are coming round to the old craft view of things, so things are changing for the better.'
Purcell, Miller Tritton, 14 St Clements Street, Winchester, Hampshire SO23 9HH (0962 868027), specialise in historic building work. Also have offices in Norwich, Colchester, London, Ely and Canterbury.
Bernard Bartrum, SPAB Fellow, 88 Hillcrest Avenue, Toftwood, East Dereham, Norfolk NR19 1TD (0362 696939), see above.
Richard Deane, 11 Ashfield Road, Salisbury, Wiltshire SP2 7EW (0722 330170), is a stonemason based at Salisbury Cathedral who also works on other historic buildings.
Glaven Pits, Holt Road Garage, Thornage, near Holt, Norfolk NR25 7QB (0263 740 848), supplies flints hand-picked from the quarry at pounds 45 per tonne.
H G Clarke, 2 High Street, Weston Underwood, Nr Olney, Bucks NK46 5JS (0234 711358), supplies clunch at around pounds 180 per tonne.
The Lime Centre, Long Barn, Morestead, Winchester, Hants SO21 1L2 (0962 713636), runs one-day courses on the history and practical application of lime at pounds 95 plus VAT per day.
'Conservation of Clay and Chalk Buildings' by Gordon Pearson, published by Donhead. Price pounds 30.
'The Nature and Subsequent Uses of Flint, Volume One' by John Lord, available from Green Lane House, Little Livermere, Bury St Edmunds, Suffolk IP31 1PY. Price pounds 5.50 inc p&p.
'The Masters of Flint' by A J Forrest, available from Terence Dalton Ltd, Water Street, Lavenham, Sudbury, Suffolk CO10 9RN. Price pounds 9.95 hardback, pounds 7.95 paperback.
'Craft Techniques For Traditional Buildings' by Adela Wright, published by Batsford. Price pounds 19.99.
The Society for the Protection of Ancient Buildings, 37 Spital Square, London E1 6DY, has relevant pamphlets.