Hi Browny , You did nt say what model valve it was. Most mira thermostatic valves use a sealed for life cartridge avialable from Mira but you will have to give them specific details such as Model and approx age as there has been variations and upgrades. Its either wear and tear or excessive system pressure, which can be resolved with an expansion vessel fittted anywhere on your cold supply sometimes called a shock arrestor avail from all good plumbing outlets.
Alan, December 2008
