Hello Geoff,
Like you I have mused over whether cruise control does save fuel over manual driving. I am sure someone has done tests, but I don't know their results.
I think it depend significantly on the type of road conditions, for example stop start city driving, or motorway, and what type of driver you generally are, do you accelerate hard away from the lights? or do you pull away more sedately? It also depends on the engine and gear box combination.
The most extensive use of cruise control is most likely on motorways or good A class roads, and here the ability of the system to sense the small changes in required load may be more frugal than a driver, but the systems is only responding after a change in the conditions, where as a driver may anticipate an incline. Where the cruise almost certainly wins out is that it keeps the vehicles speed much closer to the set speed than most drivers can, This might represent a fuel saving as most drivers tend increase their speed over long distances.
Under other conditions the opportunity to use cruise control may be limited, but it can certainly save money if it keeps you within the speed limits!
I am fairly certain that a professional driver would be able to beat a cruise control for economy over a a limited run, but could they keep up the level of concentration required? And in reality most of us are not that professional so my feeling is that they probably do save fuel, and tyres, and licenses.
I have Saab diesel with cruise, and find it very useful, even at 30mph - actually fairly critical where I live as we have a forest of Gorden Browns cash collection cameras. I also run a diesel Renault Espace, and I am trying to get cruise fitted to that as well.