Yeah I think its fine to just delete this - it happens occasionally when a DC doesn't demote properly.
Thanks for quick reply,
Just gone to delete NT-6 and now asked if I want to use delete subtree server control. Warning seems to serious for me to just go ahead and do it, so I thought I will quickly ask here before continuing :)
Thanks all. Here are the steps I followed for others who may be curious.
1. Deleted the target sites Site-Links.
2. Verified that the sites Servers container was empty, both through the MMC and in ADSIEDIT.
3. Associated the sites remaining Subnet with another site.
4. Deleted site clicking "Yes" to "Are you sure you want to delete object sitename and all of the objects it contains?"
5. Deleted Subnet.
2008 (and R2) as a DC has some superb features over 2003. One of them is simply clicking Delete on a failed DC, rather than having to faff with cleaning up using command line tools.