Further to earlier posts, Longhorn B3 does appear to be on MSDN. After a reboot of Vista, I was able to finally get to the MSDN download site, to find Longhorn is available (and can be downloaded using the FTM download manager).
Interestingly - the download speeds from MSDN are around 600KBPS vs just under 200 from the TechNet site. And as mentioned earlier, downloading via Technet is a raw download - and mine aborted half way through!
Lonhorn downloaded in under an hour - now that rocks.