You've been stung if the installer did it as a job on behalf of sky, which he did, and also took a backhander.
The photo below shows a install that Sly wouldn't do (an additional lashing kit was put on after) because they said there was no line of sight. The house shown is in the middle of nowhere in matlock area, but for the whole job, which included Sky HD and two standard boxes being installed, as well as supplying one of the standard boxes, and running all the phone lines, cost this particular customer £250 as an independant.
http://www.neroaerials.co.uk/photos/...serialNumber=1
[MOD: Oversized picture reverted to link, please resize before republishing]
The engineer who did your job would with the money paid by you and Sly would have been paid £140 in total, and you can bet your life that if any other sly engineers go out there for a service call, they won't go anywhere near it. At least with an independant, you can del with them directly and because they have their business name to look after, they will normally do the best they can for you (within reason)