Sounds like Octopus have tried quite hard and Im sure at a fair expense to improve the system.
Im not sure what those noises are though, they sound quite loud? or do you have a very sensitive microphone? Air rushing around the system is usually more of a continuous swooshing noise that can go in waves, at least from my own experience. It took me a while to get rid of the air that would sit in my hot water cylinder coil.
It can unfortunately be quite hard on some systems to get rid of all the air, even the best engineers can struggle to get rid of it in some cases - some systems seem to be harder to clear than others. The sontex is less effected but as Marko points out it doesn’t tell you if the reading as a result is in error.
It sounds like there may have been some air trapped somewhere else in the system, in a radiator somewhere? that then worked it’s way over to the hot water cylinder when it switched between space heating and hot water?
Can you try bleeding the radiators again? What is the system pressure now?