Twenty years ago, the Broadwater Farm estate in Tottenham, North London, was devastated by noting that resulted in the death of PC Keith Blakelock. At the time Christian Wolmar was a housing reporter covering the "farm". Here, he returns to find out how it has been transformed from one of Britain's worst run-down estates into a place where tenants (more than 60 so far) are now exercising their right to buy. producer Christopher Jones