Hi Glyn,
Google's organic and local algos are different. Let me give you a couple of links here I think will be really helpful in jumpstarting your understanding of Local. These are 2 projects I have been involved in this past year, the goal of which was to gain a good understanding of Google's Local Algo.
This One was organized by David Mihm and is a survey of some 30 Local Search practitioners regarding the factors most influential in the algorithm.
This One was organized by Mike Blumenthal. Five of us got together and reviewed literally hundreds of local business data sets, recording the information we encountered. This was then given to a statistician who crunched the numbers for us to help us determine which Local ranking factors are most influential.
In my opinion, these are groundbreaking and unique studies and should be helpful to anyone trying to get to the heart of the Local algo.
Miriam