There have been plenty of issues that make people concerned over food in the last few years - Salmonella in eggs, BSE in cattle and the introduction of GM foods. John Humphrys gives a brief overview of how farming has changed since the Second World War from a small scale, largely family run business to a (mostly) intensive factory business, and how this has led to our food being increasingly adulterated with fertilisers, pesticides, hormones and anti-biotics.
Now while there is plenty to get worried about in all this, and John Humphrys does present the risks well, I would have found it a lot more convincing if he hadn't given the impression that he'd really prefer it if farmers were non profit making, horny handed sons of the soil and that any sniff of profit should be ruthlessly eliminated. In this book, there are clear "goodies" and "baddies" - the goodies being the small organic farmers, the "baddies" being the EU, large pharmaceutical companies, supermarkets and the "barley barons" (a group he neither defines nor interviews).
Now there is plenty of well argued science in here. The Chapter on the history of pesticides, and how new pesticides have been introduced as their predecessors have been banned, is enough to make anyone worry and the description of how the increasing monoculture throughout Britain's arable land is allowing the spread of crop diseases (which leads, in turn, to more spraying) is well argued, as is the Chapter on GM, which is surprisingly neutral (if erring on the side of scepticism) on the subject.
Overall a good guide to the farming is practiced throughout Britain today, and if you don't mind the polemics against big business (agricultural, pharmaceutical or retail) it presents a coherent arguement about the quality of our food.