OK, so I've figured it out!
I was soooo confused that the sheep seemed somehow hungrier this year than last, since last year I was able to scoop food into the trough without any issue, by putting it in the spaces between where the sheep were.
This year, we're using the same trough, but there just aren't the same number of spaces....... it's almost as if the flock has increased by a third...... oooooh, wait a minute.....
So, I went down to the feed store this afternoon and bought a second
Paxton LF3 trough. Problem solved!

Actually, I have to say I really like this design. It's really easy to pick up and carry (unlike an equivalent sized wooden one). The downside is that they're £50+VAT, but if it's one of those things that "just works", and lasts a long time, I don't mind paying that.
P.S. before somebody points it out, I know those aren't sheep!