To quote Wikipedia (http://en.wikipedia.org/wiki/Estuary_English)
This definition is one of the few generalizations everyone agrees with. Different descriptions give the variety different features, and some restrict themselves to listing features that they don't regard as characteristic of Estuary English (although they have been said to be elsewhere). One review, from UCL (http://www.phon.ucl.ac.uk/home/wells/przed.pdf), refers to it as 'an allegedly new accent variety' (and it's not clear whether the writer means the allegation is about the newness or about the belief that it's a variety at all).
The expression derives from a TES article, published in October 1984 (http://www.phon.ucl.ac.uk/home/estuary/rosew.htm), in which the author, David Rosewarne writes
"Estuary English" is a variety of modified regional speech. It is a mixture of non-regional and local south-eastern English pronunciation and intonation. If one imagines a continuum with RP and London speech at either end, "Estuary English" speakers are to be found grouped in the middle ground."
The BBC's Voices survey, as reported in Talking for Britain (2004) says 'It all gets rather complex-sounding to describe, but if you're in any doubt, just think EastEnders' [a popular TV series - a 'soap opera' - set in east London]. The accents produced in this series (sometimes a caricature of Cockney) have had a widespread influence (especially on the young and the would-be young; some of features of Tony Blair's speech - Prime Minister until mid-2007, educated at Fettes and Oxford - are 'Estuarial') . It's a social as much as a regional phenomenon.
Browse the following links to other content related to the term 'Estuary English' from the 'Varieties and Dialects' grammar category: