Advice needed : Smart ForTwo Oil Usage

[The Dude]

I hope this is the right place to put this as we don't have a Smart section...

My ForTwo seems to have developed a thirst for oil - no sign of leaks so obviously (?) it's burning the stuff. The exhaust is quite smokey (dark) under hard acceleration. There's no gunge in the oil cap. I topped it up yesterday and from what I put in it seems to have gone through nearly a litre in about 700 miles! For anyone who doesn't know it's a small capacity (700cc) turbo engine.

I'm off down to the dealer with it tomorrow (again) but wondered if anyone could suggest what it might be and how serious it is? Just want some info so they can't try to fob me off like last time.

Thanks,

Neil