Rocking Driver's Seat

So here's a thing... I purchased a 2003 '03' Fabia 1.9 Diesel Classic estate in August 2006 from an authorised dealer - the 3 year warranty had just expired. During the first 4 weeks of ownership I noticed that the driver's seat was rocking slightly on its rails. This was repaired by having the bolts re-threaded (apparently), but within a few months the problem returned. However, although problems discovered during the first month of ownership are the responsibility of the dealer selling the car (approved Skoda dealer, vehicle came with extended warranty), and although the problem is a potential MOT failure item, it's tough luck for me now the first month has passed and I have to pay approx.