Shower wont turn off, hot water continues to run?

It's a mixer shower, the shower can be turned to the 'off' position however the hot water continues to run at relatively high pressure. I believe it's pressure related as it looks as though there is a piston within a cylinder on the hot water feed that 'should' return to the top to stop the flow of water. To add to that we have the same type of shower in two bathrooms, this problem has been on and off for a while now. Sometimes it's fine, you can (most of the time) get it to turn off by using the old hit it hard method. I believe it's either a water pressure issue or it's just a cr*p design, don't mind having someone come out to look at it for us but if anyone's got any ideas it'd be much appreciated.

Kevhl
April 2009