I'm not JB'd
What i ended up doing was uninstalling RockBand (and of course losing the World Tour that was completed) and starting again... no problems now
I think the warning on the download page about not letting the iPhone go to sleep during download is valid ... i'm pretty sure on my 1st attempt i didn't keep the iPhone awake (accidently) and that might be the problem.
Suffice to say now i can download anything again and restored all my purchases again without any issue so i'm putting it down to that.