FTP: I have been through a roof like this to the floor below because they was a copule of years old, they forgot to tell me all was reclaimed and hollow beneath, with any roof you do not know what is holding it up, there are just as many cowboy builders out there as window cleaners.
if you cannot get the fashia with a pole (cant under stand if you can get a vac to gutters fashia will be struggle but managable maybe, if not scaffold or picker, o not go on the tiles m8 as this could be the time you really wished you had listened to good advice and I do not want to say I told you so.
if they dont want to pay the price to do it safe move on it is only one job and let another mug do it there are loads about trust me