I would have guessed the bike was a Vincent HRD Series. Then again I don't know my bikes that well. My English teacher is a proud collector of bikes manufactured between 1910-1930 (or thereabout). He also restores them in his free times and several newspaper articles have been written about him. On the day the school is open for anybody to see what professions you can study for, he would always put one of his collection there and would ride it for a bit as well.
These bikes don't have the performance of modern bikes (I don't mean they are slow), but sure make up for it with style.