Nowadays, most taps sold in the UK have what is known as a ‘ceramic disc’ cartridge. This is essentially two ceramic discs with holes in that can be lined up. One disc moves with the tap handle, and the other remains stationary. A ‘quarter-turn’ switches the tap from on to off, and vice-versa. The tap body itself is purely a decorative surround to this cartridge.
It is this cartridge itself that does all the work, and generally any problems can be resolved either by replacing it, or by cleaning it.
Whilst ceramic discs are designed to last for many years (and much longer than rubber washers), we do see situations like yours fairly frequently. The good news is it is usually very easy to resolve, and doesn’t require a plumber (although the plumber should not have left your tap as it is).
All plumbing and water systems, especially in hard water areas, contain bits of debris, limescale and grit. When you install a tap on to your system, occasionally some of this debris can work its way from the pipework, and lodge itself between these 2 ceramic discs I referred to above. When this happens, the 2 discs cannot close completely, and the tap does not shut off 100%. The result is a tap that drips constantly, either from the spout, or from the tap body if that is the easiest route for the water to take.
It is important to note from the above that if this is indeed what has happened (and from experience 9 times out of 10 it is), there isn’t a fault with the product, and the situation has been caused during the installation. Most plumbers will check the tap after installation, and if it isn’t shutting off completely, they will spend 2 minutes and take out the cartridge, rinse it out, and replace it. It’s simply good practice, but unfortunately it doesn’t always happen, and the easy thing to do is to blame the product.
I would advise that the first thing to do is to remove the cartridge, rinse it through, and replace it. This almost always resolves a situation such as yours, and will avoid not only any plumber’s refitting costs for yourself, but also any collection and delivery charges from us if we pick your item up, test it and find out that there isn’t a product fault.

It might be stating the obvious, but this process doesn’t void your warranty, in fact quite the opposite: all tap manufacturers consider occasional cleaning of the cartridge part of the general maintenance that the warranty requires to be valid.

How to resolve the problem:
There are of course many videos explaining how to take out a tap cartridge, but essentially the process is as follows:
• Turn the water to your tap off
• Unscrew the grub screw holding the tap handle on. )This is usually located under a cover cap on back of the tap handle; or sometimes behind the hot/cold symbol on the front)
• Lift the tap handle off. Underneath you will see a brass nut holding the cartridge in place. Unscrew the nut.
• You can now lift the cartridge out. I recommend you soak it in vinegar, which is an excellent descaler, for a few minutes.
• Replace and test.

What to do if this hasn’t fixed the problem:
Going through the process above almost always resolves the problem on newly installed taps, or taps that have just started leaking. Of course sometimes there is a fault with the product, or if it has been leaking for a little while, or the tap has been stiff to use for a while, the cartridge itself may have been damaged irreparably.
Once again, it is important to note that the tap body is purely decorative. All taps will require maintenance from time to time, which includes rinsing out the cartridge, so there is no warranty issues in doing so, in fact quite the opposite.
Since the cartridge does all of the hard work, it is never normally necessary to replace the whole tap. In addition, a cartridge swap is a 2 minute job, whereas replacing the whole tap generally requires a plumber, and more time.
If the above process didn't fix the issue, please come straight back to me, and I can arrange a replacement cartridge, or discuss the issue further.