If it is a cheap mixer from B&Q and it has a ceramic cartridge in it .. the chances of repairing it are fairly slim. As TB say's to buy a new cartridge you need to be able to identify the make of tap, as well as the model.
When you go to buy a cartridge the problem is they are all different lengths with various sized heads on them. Not to mention the number of splines.
.. and B&Q will have discontinued that range.
A great plumber once told me .. "when you have a problem with ceramics, just fit new taps. Because by the time you have faffed about trying to find a cartridge you will easily have replaced the taps"..
That has been my rule ever since ... if the head of a ceramic tap isn't loose, just replace the whole thing.
If you want to identify which one is leaking, check under your sink, and hopefully there will be a service valve on each supply pipe.
Turn one of them to the 'Off position' and if it stops the drip, you will now know which one is leaking !