I'm assuming your shower unit is part of a bath mixer tap if so it sounds like the seal rings on the divert plunger on top in the centre of the faucet are worn so instead of closing off the faucet fully it leaks past the seals. they are fairly easy to replace.
lift the divert plunger up and insert an open end spanner on the flats of the shaft/top disk nut turn it anti clockwise to undo it remove the whole diverter unit and check the state of the rubber seals[Mine has two, one large near the top and a small one at the bottom.] I'm assuming yours is the same. I replaced both my seal rings and got a nice flow of water from the shower head.
Check in the plunger/diverter housing to see if any of the seals are left on the seating. Take the unit into a plumbing shop and ask for the seals for that diverter if that works get a spare set as they might go out of stock by the time it goes again.
When you replace the unit put vaseline on the thread that helps to stop lime build up which usually make them hard to remove. Hope this helps!.
Bob
bob Whiting, January 2005
