If your bath tap is gravity fed, from that tank in the loft, the pressure should be the same as the hot - presuming the tank is full of course! If the bath tap is fed at mains pressure the flow should be much stronger than the hot. So, you could have a washer problem in the tap - maybe its swollen and blocking the flow, or you could have a slight air lock, but I doubt that one.
John, May 2009