Brakedwell, thanks for that, however the Britannia preservation site lists 32:
bristol-britannia.com
The 15 plus 5 partially completed that I originally quoted (and is widely quoted on a range of websites and in a number of publications) has been kicking around for at least a couple of decades and is patently wrong.
Just why there is so much confusion is difficult to see as the type was built well within living memory and was well documented.