have a look undernaeath you should find a valve you will need a screw driver turn it untill the blade of the driver is in line with the pipe ,some one may have throttled it down to decrease the flow and it may have blocked up with lime scale
hope this helps john
Sounds like an air lock.
You need to connect your mains (cold) water tap in your kitchen to the tap in your bathroom by means of a hose pipe and then open your kitchen tap first then your bathroom tap second to force mains pressure through your tank fed cold bathroom tap which should then clear the air lock.