I had the same squeak coming from my driver's seat as you have. Lexus Glasgow acknowledged it straight away, and said they'd get an upholstery company who specialise in cars to fix it. And they did - when I got it back they'd replaced the foam, so no more squeaks, it feels the same and you'd never know they'd had the cover off. Funny that one dealer will take this approach, while the other will replace the whole seat.
I have a creaking drivers seat,started about 2 months ago,seems worse in the morning.What's actually causing this?