Revitalising this topic since I now need the same advice
The above is useful. I've seen quite a few reviews say that the Ikea Markus is good, although it has a leather(ette) seat, which I don't like (because you stick to it on hot days with shorts on). And I know about the Aeron, obviously, and @Okaydave is not kidding that the price is a major downside, even given how much I sit in it. (And it's two decades old! Hasn't chair technology moved on?)
I'm also gonna drop into Total Back Care on the Hagley Road, I think, although I'm sure they will say something along the lines of "here is a chair which perfectly ergonomically matches your spine curvature, that'll be the same price as a cheap car, please".
More suggestions are welcomed. (Is there anywhere in the city centre that does this stuff?)