Hmmm as the steering bearing is checked and adjusted at the 6000 mile service, could it be possible that they nipped it up a fraction too much when serviced I have done that in the past with similar results as you described. If it is not the case why did they not pick it up when the bike was being serviced as the steering head bearing is to be checked at every service.
unless the bike has been badly abused or neglected etc I think I would be asking more questions or take it up with Yamaha uk
Good luck