"... they can renovate the attic space but they cannot add dormers or roof lights or alter the actual roof in any way. "
Not necessarily.
If the top flat also owns the roof, they can alter it as long as access rights are protected. So if say the hatch to access the roof is on their part of the roof, they leave that clear. If it's on another flat's part of the roof then as long as all flat owners can gain access the attic can be built into.
Full disclosure: I know all this because I live in a top floor flat and extended up into the attic space. This was after the 2004 act came into force.
My advice is, before even thinking about consulting a lawyer, get a copy of your title deeds and read them. Lawyers get expensive very quickly and you'll be wasting your money if there's no grounds for action.
For dormers, planning permission may be required depending on the area, where they are in the building and whether they raise the height of the roof. It costs nothing to lodge an objection. However the owner may get permission regardless, especially if neighbouring buildings already have dormer windows in the roof.