[Ronald G. Stephenson]

As most know, a windscreen is made much like a sandwich, and although the techniques have improved over the years, it is still a three-piece affair. Tempered glass is used in side and rear windows and is simply tempered to shatter into almost harmless little granules. Windshields however, are expected to keep you in the vehicle when involved in an accident. The layers of glass and clear plastic separate over time and moisture enters and is trapped along with other matter.
There just might be some new old stock replacements out there, and I would first check the web and known Bristol spares places. In the alternative, making a replacement is not out of the question, especially since you have a pattern. There are glass specialists that can make windshields in any configuration. If you find an old, used screen, chances are it will be no better than the one you have. This is one fault with owning a unique, low production car. If the maker does not specify a windscreen supplied for a number of other makes, it becomes a sticky wicket for the poor owner.