Gertie - that sort of thing is hidden on pendrives - they know Dad is too much of a geek for them to get away with it on the server.
AB - too late now, all are gone, but you confirm what I saw, which is that logs aren't large files. there must have been something in the /tmp area which either was genuinely huge or was erroneously reporting itself as such.
Grateful to all for the very valuable help.