I think i have found a work around for this .....

It would appear that when you 'drill down' into a 'on demand sub-report' the the crystal Active X viewer discards the currently loaded printer driver.... selecting the 'Printer Setup' button and clicking 'Ok' loads the driver back into memory and will allow the sub-report to be printed ok with no error messages.

Currently have this logged with Crystal Decisions in London and hopefully they will come up with a proper solution to this problem.