Low hot and cold water pressure?

Hi, can anyone help me??? I have recently put in a new kitchen and two new bathrooms in my house, and now having only drained the water out a couple of times have very low water pressure all round the house. The central heating is fine. I went up into the loft to check how much water was in the cold water tank, and it was full. My house is a two storey house so would have thought there would have been plenty of pressure there to get rid of any air lock etc, however, the water is simply trickling out of all the taps. My system is fed directly from the tank in the loft which runs into a hot water cylinder which in turn feeds the hot taps. I simply find it strange that all of a sudden, having not turned any other taps or valves off (except of course the mains tap under the kitchen sink) the pressure has all but disappeared. Can anybody advise me as to what this problem is. Thankyou. Rob.