If you're a reader of ''The Observer'' then you'll be familiar with the writing of Nigel Slater who is possibly the pre-eminent food writer of this generation. I've long ranked him alongside Elizabeth David and Jane Grigson in terms of the depth of his understanding of food and the clarity of writing. Each Sunday sees me turning to ''The Observer Magazine'' for another perfectly-presented and thoughtful piece of writing accompanied by a couple of seasonal recipes. In ''Eating For England'' Nigel has given us a collection of his writings loosely based around the theme of 'England' or 'the English at table'. Some are a few pages long - others as short as a brief paragraph.