Hi bond, I have the same problem but have discovered the bathroon cold water pressure kicks in if I push the tap harder to turn it on. Its a mixer tap where you push the level up to get water and left or right depending on if you want hot or cold. We have a pressure booster thing in the airing cupborad which doesn't seem to kick in if you turn the tap on slowly, only if you whack it on with some force! However the kitchen tap always works fine. Must be something to do with how they're plumbed in!