Kirsty - maybe a bit mean, but have you tried latching Jem on when he's not particularly hungry, keeping him there just til you get a letdown then whipping him off and pumping? That way you could tell if it's time for a new pump or if you just aren't getting anything out (ETA - doesn't mean that there isn't anything there, just that you may not be getting a letdown). Manual pumps never worked well for me.
