I own a diesel Audi A3 that drives absolutely fine for the most part. However, when on the motorway, after an hour or so of driving, the clutch get very 'loose' (i.e. had to raise foot very high to make it work) and there was a strange smell. I stopped for a while and it returned to normal, but I’m now worried. My local mechanic says he can't look inside the clutch and must either replace or leave it. Do I need a whole new clutch system? Is there any way of diagnosing the problem?