M5 high oil usage - help!

My M5 has developed high oil usage and by high I mean 1 litre every 800 miles.  It also uses more oil (i.e. I have add a 1/2 litre by the time I get home) when I have driven the car in 'sport' mode for about 20 mins.  It's almost dearer to fill with oil than it is petrol!  At one stage I had driven about 4000 miles and it had consumed 7 litres of oil!  There are no noticeable leaks and no oil on the drive etc.

Anyone have a similar problem or know what the problem could be.  Car has been checked out at 2 different BMW dealerships, yet they seem to give me that look that is probably reserved for the mad half of society i.e. they don't really believe me!   They tell me that the M5 typically uses 1 litre of oil every 1750 miles, but talking to one other colleague of mine that had one he said that his used about 1 litre every 3-4000 miles. 

I'm convinced that it is something major that is proving difficult to uncover.  The car has extended warranty so if I can identify what the problem is then I can get it fixed.  Can anyone shed any light?  Thanks.