I'm mystified by your problem! I use James Martin's recipe which is similar to yours but uses rather less milk. The recipe also states cooking them for 30 mins, which I find works. Mine always rise spectacularly! The advice about not beating it is good. I roughly mix everything together with a fork.. Any lumps will disappear if the batter is rested for at least an hour. Preferably longer. Could it possibly be the milk substitute you are using? I use almond milk quite a lot (but not in Yorkshires) and I have noted that lecithin is added to it. Something like that might affect the batter, I suppose.