No cold water floweth

What a helpful assistant in Wickes when I went to get a pack of washers for this mixer tap in the kitchen.

He explained everything that could be wrong with the tap and showed me exactly what to do... but when I tried it I couldn't budge the head gear nut at all!

So I've covered it in WD40, put the top back on and will have another go at it tomorrow... except that when I turned the tap on afterwards to check if any water was coming out of it now, there was but the pressure wasn't quite what it should have been... although half an hour later the flow had stopped completely again. If I can't manage it tomorrow, I'll ring a plumber.