I have got quotes etween 1700 and 800 to replace a velux roof window. Some say they will need scaffolding, some say they can just use a ladder. Others say it is more expensive because it is a slate roof - tegral slates - and that the flashing has to be replaced with each row of slates when installing the window. Help!! I don't know which to choose. All except one are 4 and 5 star velux installers.