Both of the above part answer the question. The Government produces a handy Guide to Planning Permission for Householders that you can download here.
http://www.communities.gov.uk/index.asp?id=114 4635 In Section D you will see that you can have a fence up to one metre high next to a Highway (or next to a footpath which is next to a highway). You can have a fence up to 2 metres high between you land and another piece of land in other situations. Anything more than this would require Planning Permission to be granted. The above is not true if your property is Listed. In a few situations the Planning Authority may have removed part of your 'Permitted Development Rights' to erect such a fence without PP - such examples would typically be if you live in a Conservation Area. There is no truth in the bit about how far the fence must be maintained away from the downstairs window of an adjacent dwelling.