How to replace bath tap cartridge?

[JohnBoyII]

I'm got a bath tap with a shower mixer (like this eBay bath shower mixer

http://www.ebay.co.uk/itm/Cherry-Ch...t=LH_DefaultDomain_3&var=&hash=item3a9012cdb6

but with separate knobs for the hot and cold water).

The tap is dripping, how do I find which (hot or cold) tap is the problem so that I can replace the right ceramic disc cartridge?

 

[TeaTime]

have you got isolating / service valves fitted to the hot and cold supply to the taps?

If so, shut one away and see what happens.