Complex sentence: The distribution of these structures across the British Isles is not uniform, however it is not understood whether or not this is function of different rates of survival rather than a result of their concentration being localized to certain areas at the time the structures were raised.
Split into simpler sentences: The distribution of these structures across the British Isles is not uniform. However, it is not understood whether or not this is function of different rates of survival rather than a result of their concentration being localized to certain areas at the time the structures were raised.