A Wiki quote from Lancashire Uni researchers suggesting that Cockney came from the Kentish and Mercian accents. I would have thought that Essex would have influenced London speech as well.

"The London dialect is really, especially on the South side of the Thames, a perfectly legitimate and responsible child of the old kentish tongue […] the dialect of London North of the Thames has been shown to be one of the many varieties of the Midland or Mercian dialect, flavoured by the East Anglian variety of the same speech […]"

I could never distinguish between north and south London accents myself in all the years I spent in London. Virginia, do you know if there are any recordings of a distinct Middlesex accent?