I have the same tap. With the same problem.
They’re crap :-)
To replace the cartridge, you need to prise out the little red/blue plastic bits on the handle. No matter how careful you are, you won’t be able to do that without damaging them.
Once you get that off, there’s a screw underneath. Remove that, then you can prise the handle off, giving access to the cartridge.
Cleaning the cartridge, and maybe a smear with vaseline, will maybe help in the short term, but you’ll have to replace the cartridge. The generic ones from B&Q come in packs of 2, in several sizes -one size is about a millimetre longer than the original ones. It’ll fit, but the tap handle will sit slightly higher. Not noticeable if you’re changing both cartridges.