> the maps I've looked at on the above site were all very slow to update and looked pretty nasty 
I don't think you looked very hard. Or you were unlucky. The Dorset and Hampshire GIS mapping systems will go down to 1:2500 scale (no missing '0'; I don't mean 1:25k), and present very detailed mapping.
As Parky says, this is the Definitive Mapping of Public Rights of Way. As the name suggests, it's THE definitive source of the routes of PRoWs. OS mapping is always out-of-date, the moment it's printed. In theory, this online Definitive Mapping should be kept completely up-to-date, so you can check where the PRoWs should be today.
> Cap'n - well done, brilliant site.
Hey, don't thank me; I just found it... Thank the enlightened Local Authorities for making the data available, and Geograph for collating the sites.
I'd encourage anyone who can't find their local mapping to go looking at their local authority website, and try to find it. And then send details for Geograph. S.Yorks, for instance; does that still exist as a county, or is it now broken up into a lot of Unitary Authorities (Sheffield, for instance)?