As that's just an ordinary 2-supply tap, not a 3-way filter tap, which would have a filter cartridge, I assume you are talking about the internal valve cartridge.
I see the tap is just £69 at the moment, so replacemnt may be your best option.
Failing that, you may have some luck with the company I believe to be the OEM supplying Homebase.
Try looking at the website of Paini. I think the item you have is either:
http://www.paini.co.uk/products/0/kitchen-taps/pilot.html
or possibly
http://www.paini.co.uk/products/0/kitchen-taps/atomix.html
Cartridges are here:
http://www.paini.co.uk/products/0/spares/cartridge%20valve.html
But Pilot is so cheap it may be even more ecconomical to get a new tap with warranty :-)
X
February 2015