It was just touching the red, I stopped at at a shop and on restart it has risen a couple of mm! It has actually risen to a level appropriate for a few days ago.
Definitely not my imagination - I tapped it a few times to see if anything would happen, but no.
All fuel tanks have dividers, to stop the fuel from slushing around and giving different readings, I would not trust a gauge like that. Best to fill up and keep an eye on it.
Thanks - didn't exactly park on a slope but half on a kerb - a fairly steep one - so combined with the dividers, that might acount for it. I'll see how it looks this evening.