Safari won't open from one network client
Alas I never did figure out the cause of this. Due to the nature of where the machine was, I could only work on it for a few minutes at a time in a "test - didn't work - go away and try the fix from a SSH terminal in another room" sort of way. 

For the poor unfortunate soul who finds this in the future, the only thing that seemed to fix it was installing a security patch that was waiting to be installed. Issue just completely went away after that and I'm still non the wiser as to the cause.