Anyone know if it is normal for drivers seat on 3dr Astra SRi to move when going over speed bumps or other similar obstructions (or even just when getting in/out of car)? My car is only 11 months old and whole seat moves upwards/forwards about half centimetre and is bloody irritating. Tried other delivery mile SRi 3 door at local dealer and this appears to do exactly the same thing. They have also checked mine and predictably say it is "normal". Passenger seat however is rock solid. Is it perhaps linked to seat height adjuster?