Thanks for the replies. We've replaced, as far as I know, all the O-rings in the general area of the main fuel filter except the pre-filter O-ring. Will tackle that next to see if that clears it up.
Should there ever be any air bubbles visible in the line going into the main filter at any time for normal operation? If not, then a seal is bad and the system is sucking air. Secondly, can the prefilter O-ring be sucking air yet not leaking fuel?
Sorry if I've asked any redundant questions answered in other posts, but my car is unconscious and not responding.

Steve