Edit: sorry for lowering the tone, I just found this post extremely funny for some reason. Hopefully, someone will actually know of a reputable company in Bristol ... no, no ... I'm going ...
Just so you know Zapy, its called Window tinting, or Tinted Windows.
Beware though, as tinting any windows ahead of the b pillar(drivers ear)normally falls outside the minimum visibility legal requirements.
The windscreen must let 75% of light through, it's 70% for the front side windows.
Personally, I'd leave the front alone.
Tinting the rear windows in an estate car can be useful to keep dogs cooler (but this does not mean one can leave dogs in unattended carswhen the weather is warm)