[see update below]
They rear livestock, plant crops and build new farm buildings using just the technology of the period. Ploughing is done with an iron plough and a shire horse. Clothes are home made and meals are cooked on an iron hearth. The cast wear the appropriate clothes and, when they need specialist help, so does anybody else who comes to the farm.
During the Victorian period a lot of new iron machines started to be used in farming and it is easy to see how they must have greatly increased productivity and made farming easier; though still terribly hard work.
It is all very fascinating and the kind of program that reconciles me to paying my TV licence fee.
After a bit of Googling I discovered that the same independent production company had made an earlier twelve part series, called Tales from the Green Valley [poor title, I bet they now wish they had called it Jacobean Farm] , about life on a farm in the 1620s. Both series have books and DVDs so I sent off for the Green Valley DVDs and I am working my way through the episodes.
Though only just over 200 years separate the periods being portrayed the technology is very different. On the Green Valley farm ploughing was done with a not very efficient wooden plough pulled by a pair of oxen. On the Victorian farm they used a very efficient iron plough, doubling or trebling productivity. On the 1620s farm they would have been living on the edge of survival and illness or a run of bad weather would have been very dangerous. On the Victorian farm they were starting to get their head just a bit above water.
Update December 2009
The Victorian Farm series was such a success for BBC2 that they commissioned a three part Christmas 2009 series which has the three characters returning to the Victorian Farm at Acton Scott.
The BBC has also commission a fourth series called 'The Edwardian Farm'. I understand this has the three main actors from the Victorian Farm, but is set in a different location. Edward VII reigned from 1902 to 1910, but the period to the end of the First World War is sometimes considered part of the Edwardian Era.
I am looking forward to seeing it.