That sucks mate... Sounds like you've got a bit of a lemon.
I doubt fair trading would get you very far because Subaru have been covering under warranty & "technically" looking after you, even if it's not to your standard. If they weren't, then it would be a different story.
If I were you, I'd get it back (Run it in) & just get rid of it if you can (IE: If you can afford to do it)... Although it is not uncommon in the 2.5's to have issues such as this, what you've experienced isn't right IMHO.