This is the reason I sold my Focus.
It was using about 1.5 ltrs every 400 miles.
No external leak, no black smoke, no power loss......took it for a pre-emission test and it passed with flying colours even though the inside of the exhaust was full of black carbon.
What oil is been used? Proper approved dealer oil?
Any power loss? Try clean the inside of your exhause pipe out as much as possible.....(wear gloves
) then you can see if builds up again quickly or not.
Also you could put a large piece of cardboard under your engine bay after a good long drive overnight. see if there any drips on the cardboard.
Other things to check:-
PCV system. (not sure where or how the Seat one works - on my Focus there were two seperate pipes for the PCV system. One going from top block to air filter housing and the other was lower down near the exhaust manifold but on the front of engine, this had a one-way valve attached to it.) Make sure any PCV pipes are clean and clear, check any valves are opening ok.
You can always send me a sample of oil from your engine to test, I can see ifthere is any blow-by happening or any wear inside that may be a result of worn rings etc.