mixer taps?

i have a mixer tap on my kitchen sink,
two tap heads unscrew & are removable,
there is a nylon washer around half-inch long and has grooves that the tap pushes onto,
these nylon washers need replacing as when tap turned it just turns&turns as its not turning tap outlet,
im not sure what make of mixer taps are as they was from mfi kitchens arond two/three years ago,
help gratefully received,