srw
It's a bit more complicated than that...
Bigtallfatbloke said:but from where I live (which may be the issue) I see no useful routes...say from Essex south tot he coast or west & around london etc
From west of London (Rickmansworth), there are a series of mostly flat traffic-free paths via Watford, St Albans, Hitchin, Hertford to Ware. I haven't carried on eastwards, but they do into Essex. Presumably that also means you can use them to go West around London!
They're not perfect - I'd take a map (the 1:100,000 OS maps are fine) - but without sustrans they wouldn't be nealy as findable.