There is a company here in the North West called Cooltint, they've been going for years (they do a lot of office windows as well) we've used him a lot and he uses a spray on the inside of the rear perspex window. He tells me that the reason film shouldn't be used is because the plastic emits a very low volume of some sort of gas which will cause the film to bubble over a period of time. Whatever the technical reasons are, he comes out to you and does a great job. The guy i deal with is called Tony, 07949 110255. This has to be a better solution than cutting out the perspex and bonding in the FR/Stylance window which often still isn't dark enough.