A 'Sturbridge Village' in the Sussex Downs

She looks like a 14th-century farmer's wife, sitting on a simple wooden bench in front of the smoking hearth at Winkhurst Farm House. But she's one of the modern volunteers at the exhibits at the Weald and Downland Open Air Museum.

In the heart of the Sussex South Downs, this museum is a treasure hunt of old buildings dotted over some 40 rolling acres.

The overall Weald and Downland Museum, founded two years ago, exists to rescue and rebuild vernacular buildings - from cattle sheds to carpenter's shops , cottages to pugmills, market halls to farmhouses.

Winkhurst itself is a late 14th- or early 15th-century timber-framed building , donated to the museum to save it from drowning. Had it remained at its original site, at Chiddingstone in Kent, its tiled roof long since would have vanished below the rippling waters of what now is the Bough Beech Reservoir. In 1969 it became the first structure to be reerected at the museum. Before Winkhurst was dismantled, it did not look very much as it does now. In the 17th century a wing had been added, and the timber-framing was tarred black and filled in with brick.

At the museum only the two-bay medieval building has been reerected, the exposed timbers have been palely limewashed, and the infill is ''wattle and daub'' instead of brick. The roof tiles soon are to be replaced by thatch.

All these differences are aimed to make it look as true to its original medieval self as possible. The hall with its open fire - literally in the middle of the floor - and stairs leading to a single upper chamber indicate a simple way of living. There is no chimney. The smoke escapes in swirls through a triangular opening at the top of the gable. The windows are unglazed and just divided by wooden mullions.

Despite its simple appearance, this house was constructed with great skill. And today, even separated from its setting, it has tremendous atmosphere. The same is true for the 27 other buildings at the museum, particularly the other houses.

The period covered by the exhibits is long, from the Middle Ages to the end of the 19th century. Rescued from all over southeast England, the buildings reflect the breadth of their geological, and consequently architectural, differences. Timber-framed buildings are most numerous, for example, in the Weald, which is clay and sandstone, and where oak trees grew in abundance.

The Downlands, on the other hand, are chalk, and flints were the normal material for stone walls. One fascinating exhibit is a house from Walderton in Sussex. Externally it is of 17th-century flint and brick; but inside it contains the remains of a 15th-century timber-framed building. The reconstruction at the museum is intended to illustrate the way it changed, from a medieval to a 17th-century house.

The Market Hall from Titchfield in Hampshire was in a derelict, neglected state when it was rescued. It has required not only reconstruction but major repair. The problems involved are shown in an explanatory display inside this delightful building.

Rural buildings are the major part of the collection. There is a marvelous little granary of the early 18th cmnTury, raised off the ground by 16 mushroomlike ''staddle stones.'' There is a working watermill. There is a stable , barns, a 17th-century treadwheel, a smithy, a saw-pit, and even an unpretentious carpenter's shop from Windlesham in Surrey, little more than a timber shed, built at the end of the 19th century.

The museum owns about the same number of buildings in storage - most waiting for the financial sponsorship needed to rebuild them.

Reconstruction of one already has begun - a classic medieval hall-house of four bays from North Cray in Kent. It is expected to be completed by the end of 1984. Next will be a tiny Victorian schoolhouse, for six village children. A mere 20 feet long and 15 feet wide, it is built of flint and stone. It will be furnished like a typical classroom of the time.

I asked Christopher Zeuner, the museum's director, if he had had any difficulty in finding the craftsmen needed to rebuild traditional houses - to thatch, to split hazel rods for wattle, to tile, lay brick, to join, and so forth. He said he had not: ''There's a wealth of skills.''

He added that it is ''a bit of a myth'' that these old crafts are dying out in Britain. He does not have to range far and wide for available skills; but he does feel strongly that the craftsmen employed should be paid today's wages.

He told me that the museum carries out a continuing program of planting - native trees such as beech and hornbeam) - to ''gently modify the landscape.''

To the south of the museum is a woodland area, partly open to the public, which includes a reconstructed charcoal burner's camp.

A garden has been started at the early 19th-century tollhouse - another of the exhibits; further gardens may be planted near other buildings.

During July and August the Weald and Downland Museum is open every day; from April 1 to Sept. 30, every day except Monday; and from Nov. 1 to March 31, on Sundays only.