No I hadn't. However I have just decided to start afresh. We had other issues just with CS itself which while we got it working nobody could tell me how.
I'm getting the whole site re-done from scratch so we'll install the shop again - from scratch.
Our issue seems to be centred around the fact that anonymous browsing of the site presents the 'IUSR_...' credentials through to SQL Server rather then the asp.net one. I have a vague recollection that the asp.net user is something to do with IIS5 but I'm past caring. This site was supposed to be online by now.
Thanks once again though for your extremely excellent support!