firstly, i would never bolt anything into a roof, as there is always the potential for leaks. a big concrete base should really be constructed on a flat area of the roof. so the original ghuy charged you cor fixing a dish that was installed due to his incompetance...hmmm
secondly, if you took pictures of when the dish down and any damage to the roof or dish you may be able to claim for a new dish from the insurance company (it may be a bit too late now but it may be worth a go)
thirdly, 900 euros for a 1.8m dish seem a lot of money...especially if it is just the dish that needs replacing. personally i sell 1.8s for 700 euros installed. i will coceed that it is difficult to get just the dish itself from suppliers without the whole support rig, but check what the company quoting 900 charges for a full installation...then knock off 50 to 75 euros for the lnb, 50 - 75 euros for the cabling.
a damaged dish can perform less well, as the knock on the bottom of the dish may "squash" the dish also, so the sides may be bent.