our mira 723 shower wont stop dripping, how can i fix it?

When the large round flow knob is turned completely to the right / off water continues to drip / run out of the shower head. Our shower is a mira 723 installed in 1989 / 1990 and the control knob is white plastic. We live in a hard water area and have already replaced 1 cartridge ( cost £100.00 + ). Can anyone help, please ?