Hi Foz2,
IMHO it is a combination of aptitude and practise (as you have guessed!). The spatial skills can be tested (to some extent) by an ATC simulator game (used to be called TRACON). This will give you something of a feel for radar control anyway!
I have seen both naturally gifted ATCs and moderately gifted hard workers make it fine. I have also seen very intelligent people that are unable to grasp atc at all so I don't think a direct link to intelligence is it.
Hopefully others will comment so you have a variety of responses
I consider ATC to be a way of thinking...
Blue (great question BTW)