Indeed, Sen. I love language, accents, dialect too. And, being Eas' Lunnon myself originally (though not, technically, a cockney), I am well aware of the lazy f (hence my 'fink (sic)' above). But I don't think that stems from A/S. I can't think of any words, off the top of my head, where an A/S th has morphed into a modern f.

Half-right, Thompson. Thorn (Þ / þ) is one, but there is also Eth (Ð / ð), Eth being the softer 'th'. Given her description, I think Sen is probably thinking of Eth, rather than Thorn.