Look in your loft - you may find two water tanks one big and one small - the big tank has water coming in at the top and the outflow at the bottom - this outflow pipe should have a stop tap on it. That's the one you need to turn off.
Some WC cisterns have an in-line screwdriver stop valve close to the cistern itself - if you have this (check first) you can turn the water off there - otherwise it's up to the loft.
Good luck...
Peccavi
January 2010