My understanding is that bitumen and asphalt are the same thing ie. that sticky black stuff you get under the old thermoplastic tiles and never quite dries out. Mastic asphalt is asphalt with aggregate mixed in to it and will set hard. It is usually laid up to about 30mm (I think) on a fibreglass membrane so is a floating installation. It is a much sort after sub floor by carpet fitters who like to nail gripper into it and completely ruin it for the purpose for which it was intended.
