We too have awkward walls and ceilings. High ceilings too, and lots of woodwork (picture rails etc). We had our hallway stairs and landing redecorated by a professional who did a fantastic job. There are no bubbles, and only one place where the seem doesn't quite meet (very awkward spot, it's been blended with a thicker basecoat). It cost us £800, plus buying our own coloured emulsion. We're in the SE.
I would ask the decorator one last time to come and fix it. You can't expect a perfect finish with awkward walls, but you can expect there to be no bubbles, that the lining paper stays in place, and for the paint to be the same shade (allowing for shadows). Imperfections would be things like bumps in the wall showing through a little, dings in the woodwork not being completely filled (our guy offered to sort all of them out, but warned it was labour intensive and would put the cost up, we quite like some of them, they give character, and I preferred the cheaper option!)
If he still won't fix the problems, I would look at getting quotes from someone who will, and would look at the legality of paying the £6k minus the cost of making good. I would also make sure I had copies of correspondence asking him to make good, and a warning about having to get someone else in.