Cool. 

Sounds like there is a way of controlling the air rad and the primary pump as a couple. The UFH will simply draw what it wants when it wants, when the 2-port is open. 

The hammer may well stop just by removing the primary pump force ( by teeing the UFH below it ) so I guess I'd start there. The issue is cost for the plumbers time if it also requires the ZV moving as I suggested. A second visit may not be economical so maybe do both out of diligence whilst he's there and it's drained down. 

From my experience, of fitting regular ZV's in the flow not the return, the only hammer I've ever had issue with has been duff actuators 'bouncing' a couple of times as they close, which created a loud water hammer sound.

I simply think it's 

a) the wrong valve

b) in the wrong place

c) and the UFH doesn't need the primary pump force as it's doubling the velocity and fighting the ZV. 

Argggh - Note to self "RTFM idiot".

I've actually spent the last week at the new build and have had chance to "tune" the heating system durint the cold spell. The root cause of the problem is that the tank pump (Danfoss UPS2) was set to constant speed rather than proportional preasure. Changing this over has cured the probelm as far as I can tell. (Either that or I'm sleeping through the noise).