My view is that Rosewarne misguided in very many respects. For a start, it's is not a new variety, it's just a standardised form of speech with Southeastern phonology. People have spoken like that for years and years.
EE retains some regional low-level phonetic features.
What MAY be new is the fact that the non-standard urban dialects are being levelled in the whole SE region, so that it is increasingly hard to tell even where nonstandard speakers come from. Rosewarne completely misleadingly tries to associate EE with certain discourse features, such as stressing prepositions and using tags. This is nonsense, and seems to be based on his dependence on local radio for his data. What we can say is that, although attitudes to it are still not positive, it is becoming more and more used in high-status occupations, including broadcasting. We used quantitative methods to study phonological features.