I got the 25mm breather and then a small bit of pipe with an outside diameter of about 23mm from B&Q. Stuck the pipe in the breather filter and the other end in the SAI pipe and jubilee clips on both ends.
Seems to be working ok for me.
if you warm up the 25mm breather filter outlet in hot water you can push it over the 28mm SAI pipe and there is no need for jubilee clips or mods to the SAI outlet.