It... isn't 100% terrible.
When my hair's down, it looks more or less like it should - but with hair up, you can see where the blending did not work very well underneath. He actually didn't do the same backcombing technique on those sections as he did on top - no idea why. But the line is pretty visible, and it is not a subtle change in colour either! Ack.
So while I contemplate whether to live with it, try and fix it myself at home, or be terribly un-British and actually go back and ask them to fix it, I decided to live with it for a couple of days and see how I feel. Meanwhile, I have noticed that my hair is now not unlike my tabby-tortoiseshell cat's.
She definitely wears it better.