is the hot water pressure at other taps in your house fine? if so then id say you need a non return valve, not likely to be air locked if the pipe going to the basin comes off of one connected to pipe that goes to another tap? ie kitchen, and being that its being fed from upstairs to downstairs you should have effiecient enough pressure, so all i can see it being is the tap not having a bs mark, as all new mixer taps with a british standard have metal going through right to the end inside the tap to solve the problem of things like the cold pushing the hot back up the pipe if they are seperated inside the tap till the end then one pressure cant push the other pressure back and it mixes at the outlet, which i dont think your tap has, it says nothign about a british standard in that ebay listing, so fiting a non return valve should solve this problem if thats the case