I could be completly wrong but my first gut feeling was Stuart Crystal, ive had quite a few of there decanters over the years and my nan is a bit stuart mad. I have seen marked stuart in a very similar shape, also the shape of the base the rounded edge (side shot) very similar to one i have in stock somewhere.
The actual base is typical for many crystal decanters which includes stuart again.
I havent seen many Stuart pieces with the amount of cutting that yours has though
sorry not much help really just thinking out loud

michelle