new ceramic tap fittings have reduced hot water pressure

[slightlyworried]

I have replaced both ceramic fittings in a kitchen sink mixer tap. (We have very good mains water pressure.) Now the hot tap now seems to have very poor pressure (cold is fine) On looking at the replacement glands, the hole seems smaller on the new taps than the old. When I bought them however there were not different sizes to choose from (at B & Q) , so is it that the original with the tap were bespoke, or will there still be larger ones available and I can hunt around to try to find ones with similar size holes?