uk20.. Don't worry; you're not the first one to ask the question. I showed a BA Command Course (potential Captains) around Heathrow ATC some years ago. One of them whispered: "Where's the computer that tells you what to tell us?" I simply pointed at my head and he was taken aback!
Controllers are highly trained individuals in many aspects of ATC. Radar Control is but one and it is a serious skill. There are, of course, certain procedures to be followed, eg points at which aircraft may be descended, etc., but the basic instructions are decided by the individual ATCO with no assistance except his training.
NATS have tried various types of computer assistance to make the spacing task at busy aerodromes "better" but none yet as been able to beat properly trained and experienced controllers.
Hope that helps...