This finally explains something which has been bugging me for ages!
I'll turn the lumbar support on to full, but after a while I can't notice it any more. I'll turn it all the way down then back again to re-set it, but was never sure if I was just getting used to it being there- as above, assumed it was some sort of plate moving out- had no idea it was a kind of bladder.
Sounds like I have a slow puncture!