As a favour to anyone else out there that may hit this one.
svn: Can't open file next-ids: The process cannot access the file because it is being used by another process.
If you see this message on your Subversion server - it's likely that your real time anti-virus software has got a hold of the next-ids file while it was being written, locking Subversion out.
Exclude the transactions directory in your repository from your AV software.