Hi everyone, I'm new to this sort of thing but I can give you my experience with the retro-fitting of cruise control on my Antara Exclusive 2.2 Diesel (2 wheeldrive).We (the wife & I) purchased our Antara in October 2013. It was 10 months old with 1500 miles on the clock having been a demonstration car. I wanted cruise control fitted and the salesman who was excellent all the way through the purchase said "we can fit that no problem but there will be a charge as the car has to go on their computer which needs to be told that the cruise control stub had been fitted." All it was, was a simple removal of the indicator stub with a special or appropriate tool (a very fine screwdriver) and replace it with the correct stub. Cost me £100 through the dealers although I found afterwards that it was a simple operation to D-I-Y, as long as you could tell the computer brain that its there.
I suppose I'm safe in the knowledge that it has been fitted correctly and should operate correctly. No problems as yet and I find it a great way to drive the car. I dont know if the fuel consumption is adversely affected by it at present as I'm only getting around 32 mpg. Whichever way you have it fitted I would go and have it done.