Ok, I've totally nailed it.

It's the naps and how late she has a nap in the day. Yesterday was good, she went down at 6pm and slept.

Today we had a nap (both her and I, I needed a snooze ) at 2.30pm - 4pm. She wasn't tired until 8.30pm. So I think that we just time the naps well, or accept that if she naps later then she goes to bed later.