Help neede to find washer and seating sets

thor

I have been struggling with my leaky mixer tap for a while (have also posted here previously about it) but cannot find a permanent solution.
The problem appears to be the hot water tap cartridge or at least something connected to it. I have replaced the cartridge with new ones but the only last about a few months before the leaking starts again.
Recently however I have found a clue about a possible solution because I noticed that if I push the top of the cartridge backward then the leak stops completely while I am holding it back. That suggests to me that there must be a gap where the base of the cartridge washer makes contact with the seating. I have been doing a bit of reading about this and there appears to be a way to fix this using a washer and seating set which can be added to the set up to make a better contact. I am desperate enough to try this but have not been able to find any such sets anywhere.
does anyone know if B&Q, Wickes or maybe someone else supplies these?

ormus

they all sell fibre (or rubber) seals. as do any plumbers merchants.
you need to measure the diameter of the one you want or just get a small selection pkt/box.