i've had this fixed now (presumably) but thought i might mention it for the record.
for a few months whenever i turned right/clockwise around corners, uturns or roundabouts my drivers seat would clunk or click. the sound and feel was very similar to when you set the seat forward or back and it clicks into place on that silder. but it was definitely in place and only did it on clockwise turns. it started doing it maybe once a week but slowly became more often til at least once per day.
on my 30000k service last week they said they tightened up the bolts etc underneath and while i have had one teeny click since, it has been almost perfect fix. and no it didn't start right after a previous service in case anyone wonders if they may have loosened the bolts for anything previously.
it was pretty disconcerting and sometimes i worried the seat was going to break.

also started around the 2 year anniversary mark so not sure if the mechanics of the seat would be out of warranty if it was more serious or if that is under the full 5 years.
anyway, hopefully no more issues there.