Firstly, I apologise if this is the wrong place to ask this question. I'm new to the site and I was just having trouble navigating through it (long day).

I have a Gen 2 Mini Cooper, and it's over revving. It happens when I accelerate hard in any gear, and the revs jump immediately from around 2,000 rpm to 5,000 rpm, and then immediately back down to normal revs again.
I have read that this could be the clutch, however, BMW said that there's nothing wrong with the clutch, and the problem has not been getting worse over time (which I would presume it would if the clutch was wearing).

Other things I have done to try and sort it out:
-Changed the air filter, checked for holes etc, in case it was to do with air pressure;
- changed the spark plugs;
- had the car serviced (where there were no apparent issues);
- ensured I only use Shell Optimax.

It may also be relevant that the car is de-catted, and there is a tiny oil leak at the front somewhere caused by a stone.

I just can't figure out what the problem is, and the dealer can't either apparently. Can anyone help? Could it be that the clutch is fine, but oil is somehow leaking into it and making it slip?