You need to replace the flush-valve seal (Also known as dual flush valve washer, or diaphragm washer) at the bottom of the flush assembly.
How to do it :
http://www.lunns.net/cisternspares/roca_flushvalve_installation.pdf
Roca Part # :
8-22403000, about £1.53 + VAT from ROCA, £7.19 from Lunns (70122 - Roca Diaphragm Dual FV Washer)
happy flushing
Dave
dave, November 2007