Thanks Isolated. Reading the instruction booklet i reckon i could change the cartridge myself. The hard bit is getting hold of one as Crosswater are reluctant at the moment as they think it's the installation at fault. I'll work on them again tomorrow.
The more i look in to this the more i start to understand and i wondered if this scenario was feasible. If the pressures have not been equalised, or non return valves fitted could the water race down the supply pipe to the shower valve and as it passes the bath tap suck some of the water back with it? This could be enough to open the cartridge just enough to allow water to pass. When the shower is shut off the change in pressure closes the cartridge again. If this is feasible then i would have thought that fitting non return valves would stop this as they will hold the pressure in the tap.
The instructions do say that the hot and cold supplies should be of equal pressure and that non return valves will need to be fitted.