Thanks Ken!
In case any other beginners are interested, I came across this very interesting read about measuring the level of light pollution using a dSLR histogram, and how to optimise the number of subs vs exposure time per sub vs total exposure time.
http://www.pbase.com/samirkharusi/image/37608572
According to this method, it takes around a minute for my background sky at zenith to reach a mid-level histogram with f/4 at ISO 800... i.e. a fair bit of light pollution compared to the 12 minute+ figure for dark sites.
I might try imaging with hundreds of 30 sec subs at ISO 1600 to see how that goes.