Hi BOAC,
fred's knowledge has been probed before - he has very little experience but is extremely keen and able to point to FCOM references. We all started with a confused understanding. If they are brave enough to ask on an open forum, then I think they deserve some help.
To me the question 'how can an aeroplane pressurise with the packs off' is a pretty broad hint.
Field in Sight explained very well how it is possible in post #13.