Kildoozer, thanks for your help looking at my setup
For anyone with this problem, the resolution here is that all sites that you wish to have in your network stats must be at the latest update level.
In my case, I never update all my sites at once in case there is a problem (I've been bitten like that before) so in this case I had updated one site, seen the network stats problem and therefore not updated any others.
After updating all sites in my network (which I must say did take a while) I now have properly working network stats.
Kildoozer and ip0li, a couple of suggestions come out of this
1. It would be most useful if the network stats page would warn me if a site was not at the right update level - I'll add this to the features thread
2. At a minimum the manual should make it clear that the network stats feature requires all sites to be at the same level.
Thanks