As from 6 April the Bristol Airport to Bath Airdecker bus is now run by First Bus as their service A4, same timetable as previously
So the Airdecker website is no longer working, for anyone looking for it- see the First Bus website.
The fares are much lower than previously- now at £9.50 single, or £16 return- so the same as the A1 from the Airport to Central Bristol- https://www.firstbus.co.uk/bristol-bath-and-west/plan-journey/timetables?search=a4