I've got so much xp, but I cannot find a button anywhere on the character sheet.
Manual has a load of references to level up effects, but fails to mention HOW to do it!

So how do I level up?

The little plus sign on the top left corner of your character portrait. Click that.

There is no plus sign, either on the little portrait, nor on the big on in character screen. o.O

Edit: Huh it IS there on the little portrait. My characters hair obscures it!