Just tried that 20min Cat Anderson note and it is a bloody excellent exercise! I circular breathed it 3min blocks. It is certainly possible to do this with a bit of hard practice. Incredibly, i never realised how much a gap i have between my teeth. As soon as i moved my jaw up a little bit my central pitch was higher. I think my jaw was set on a low note position. It certainly gives the embouchure much more support. I thought my embouchure was OK, but clearly not.
Thanks for all the useful advice on this thread.