After posting the other night I did a bit of research online which seemed to suggest that the solution to this rested with something in the hosting company's server set-up. I passed the error message on to them with the suggestion that open_basedir might need to be disabled and the page is now working, although I have no idea whether that's what they did.
It occurs to me that the error code you gave me could probably be added at line1 on a lot of pages, to clarify issues - would that be a sensible practice, do you think?
Thanks again for your assistance, Jason, as always, much appreciated.
