Thanks Silverfox for the info (I had already found Tibor's article on this which is what led me to the backup shrinkfile)
I have now cured the symptoms. I checked the physical size of tempdb (4gb) and increased the initial size of tempdb to this. I shrank the db log and data files refreshed and the negative values went.
I don't know if this was the case, but it appeared that SQL server was not calculating free space on the actual size of the DB, rather on the initial size of it (the sums were about right if this was the case).
Thanks for your help
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