it might take a little time for the air to fully escape since you have had the fuel system opened up i normally take mine for a run other wise i purge it many times
as for the slight pressure drop this depends on how your measuring is the slight pressure drop more while coming on boost ie fuel demand... or is this pressure drop les the same when engine is just idling if so yes posibble one of the stages is failing in side the pump
also when you renewed the regulator did you clean the gauze in side the cylinder head also there should of been a small oring
ps where did you purchase your regualtor i also got told from landrover to stay away from the ones on the net as they noticed they also had issues apparently the same fuel pressure regulator is used on some VW engines that run slightly different pressure.... unsure what i got told is true as i had many issues with them in the past... basically there words was they are the same but not the same but then they go on to say if i want one we will have to order it in probly on the net
is your fuel pump making any weird noises like screaming ?