here is an extract from here if you'd like to read it
"(TH): The fronting of /θ/ and non-initial /∂/ to [f] and [v] respectively: ?think? [fÃÑk]; father [fá:vë]: This has received widespread attention, both by journalists and linguists. It appears in many urban centres across Britain (and beyond: Horvath 1985, Campbell and Gordon 1996) (see the papers in Foulkes and Docherty 1999 for a summary, and also J. Milroy 1996 and Llamas 1998). It is sometimes considered as a ?London? and not an ?Estuary? feature though it is difficult to ascertain, in the absence of socio-economically stratified and sociolinguistically sophisticated analyses, how such judgements are made. Trudgill noted just how rapid the change to [f] and [v] was in Norwich (Trudgill 1988: 43)). He found that whilst none of his speakers born before 1957 fronted the TH forms, a staggering 70% of his informants born between 1958 and 1973 did so, 29% of the informants consistently."
from here