I'm a bit surprised that nobody has so far mentioned FindBin, a core module, which is intended to solve your problem.

** slaps self round the face **

Yes - I knew that I was missing something obvious and FindBin is that thing!!!

I was aware of it but had completely forgotten about this simple solution!

I've actually solved the issue by feeding lib an absolute path on the basis that the script cannot move without updating cron which has an absolute path. Given that the modules won't move anywhere else, this is probably a reasonable solution.