Shadowman,
Thanks very much for your response. The problem is exactly as you have understood. We have been frequently using the PLA material and have not used the PVA material or extruder.
when you say 'we had to remove a significant amount of brown carbonized material' did you do this using the hot / cold pull methods?
I have read elsewhere that an acupuncture needle can be used to poke through the end of the nozzle when the filament is heated to so I feel a combination of these methods should be used.
I think a spare print core is also probably a good idea.
Thanks again