Thanks for the offer of help
As far as the holes in the coverage of service areas goes, it seems to have something to do with the very variable nature of the network I'm using with incredible density in some parts and very sparse in others. In practice I've given up trying to figure why the holes are there and solved the issue by using a uniformly more rich network. Tonnes of extra processing and work but I'll end up with no functionally important holes and a more robust map. the SA polygon generator still produces weird small missing areas in polygons but these are small enough for me to 'fill' them non-contextually by merging them into their smallest neighbour.
As far as the inconsistency in SA areas when using different numbers of facilities, its still foxing me but I'm finding that network analysis is a bit fragile on my machine - and once it's kicked up an error it seems to wobble in subsequent operation afecting everything else. Thus at the moment I'm rebuilding a test network for the third time today - each time its built differently dropping edges at random, for no reason. Restarting Arc with a fresh instance and picking up a clean version of the data seems to work wonders, so it seems that I'm back to where I was back in day with arcview 3.0 - constantly saving individual steps because the application would wobble at the slightest provocation, not work or corrupt data.
james