Thanks for all your advice guys, and I'm really glad to know that my DD is getting all she needs, despite the strange behaviour.
I have found that leaning forward has been helping, but she still seems to fuss. Yesterday she was feeding for 5 hours, but it was useless feeding, 5 minutes and then agitation. I made the mistake of switching breasts everytime, thinking I must not have enough milk for her growth spurt. After discovering with dismay her usually lovely yellow poo had become green, I did some research and found that her iffy feeding pattern combined with my switching, had resulted in her getting too much foremilk... that could explain why she couldn't get full!
So with my new knowledge we are doing better, but she still fusses quite a lot and it's quite irritating, especially with the night time feeds, regardless of whether my breasts are particularly full, or not so full, and also doesn't seem to make a difference whether she had a feed one hour ago, or 2.5 hours ago..
Also, I'd like to know, if you feed with one breast for 10 minutes, take 10 minutes for nappy change and nap, and go back to that breast, is the milk at the same stage as you left it, or is it full of foremilk again?