Sky special heights team (not really a team but not thier standard installers) will only go to a height of 30ft (the point to the height of the dish), anything higher and they normally refuse. If you do get one to go higher then yes they will normally charge you extra, how much extra is between you and the person. £100 does sound a lot especially as he is also getting paid by sky
I personally wouldn;t expect to pay more than £50-£60 extra as that would cover any extra time and equipment needed