Over the past decade or so, it has become increasingly common for some UK English speakers to stop sounding "th" in the normal "Receive Pronunciation" manner. But recently I was listening to a spokesman discussing the gang culture in the Borough of Hackney (the northeast part of London) and suddenly realised that, not only was he never pronouncing "th" properly, but also that his mispronunciation varied according to the word.

So, words like "this", "that" and "those" became "dis", "dat" and "doze".

But other, similar words, such as "things" and "thoughts" became "fings" and "foughts".

So the phrase "I think those things are OK in theory" became "I fink doze fings are OK in feory" - which one would have thought was more difficult to say than the correct pronunciation.

Has anyone any idea of the rules for deciding whether "th" converts to "d" of "f" - or is it just something that is learnt - as are so many of our strange pronunciations?

Then there's the "v" sound to contend with. A word like "weather" becomes "wevver".

Off the top of my head it looks as if the voiced "th" of then/this/they etc is being replaced by "d" while the unvoiced "th" of thin/thick/thing etc is being replaced by "f" but I'd want to listen to a wider sample before being sure about it. A voiced medial "th" is being replaced by "v" and, although there is no example here I suspect an unvoiced medial "th" (such as "ethics") would be an "f". Voiced and unvoiced terminal "th" probably behave in the same way as medial. Bath/Bathe -> baff/bave.

Correspondence of /f/ for /θ/ and /v/ for /ð/ is a common phonological feature of Cockney (and has been going back at least 100 years or so). What's changing is the place of articulation, from dental-alveolar moving forward to labial-dental; the manner of articulation (fricative) has been preserved, as well as the voiceless-voiced distinction. It may be an example of Mockney (link) or Estuary English (link). Your best bet is asking a British phonetician or socio-linguist.

The substitution of /t/ for /θ/ and /d/ for /ð/ is common enough in non-native speakers of English trying to find phonemes in their native languages which are close to the dental fricatives. I've heard this substitution made in Irish English (mainly stage brogues), but also in some of the UK South Asian population. Another substitution one hears is /s/ for /θ/ and /z/ for /ð/ in Francophones.

Something like this also might have happened in Latin, with the Proto-Indo-European *dh becoming f, as in Latin foras "out of doors" from *dhwer- "door". It also happens to words borrowed into Russian AIUI, for instance Theodor in Russian is Fëdor.

Didn't a similar phenomenon occur in The Bronx? Was their peculiar accent due to a shift from Dutch to English phonemes?

Didn't a similar phenomenon occur in The Bronx?

Yes, dese and dose for these and those.

Was their peculiar accent due to a shift from Dutch to English phonemes?

I'm not sure how old the Bronx accent is. It seems to have been around since at least the talkies (late '20s). Maybe a bit earlier in Damon Runyon stories and the like. My feeling is that the Bronx was pretty rural up until the post-Civil War period.

Many young (or would-be young) urban English folk speak in their own argot which is a mix from a lot of sources, reflecting the multi-racial makeup of London and other large cities. Some of the "outside" influences are from African and Indian languages, and Afro-Caribbean pronunciation. There is also a strong influence of American "Gangsta" rap. Could the latter be causing the shift to 'dese' and 'dose' for 'these' and 'those', etc?

a shift from Dutch to English phonemes

Asa, I found somebody who agrees with you (link). And that article led me to Dr Jochnowitz' delightful website (link). And it ties in with what arnie is suggesting: that many of these "accents" or sociolects start out as a mixture of language groups coming into contact with one another.

Ah, now I'm reminded of the famous play, "The Melting Pot:" http://www.vdare.com/fulford/melting_pot_play.htm
Check out the accents! The play was first staged in 1908.