Ok, I think I understand your problem. Please forgive me, because I had to look up some terminology in your post because I see that you live in the UK. I live in the USA, so I wasn't quite sure of some things.
It's my understanding that the groegg is a temperature gauge? 18 Degrees Celcius is about 64 degrees Fahrenheit, which is a bit too chilly for a baby. If she will not stay in the sleepbag (which my baby won't either), then why don't you try layering? I dress my 15 month old in an undershirt and then footy pajamas. That way if she does kick her covers off, she still stays warm. She has slept through the night since she was 6 weeks.
Can you try keeping the temperature warmer also? We keep our thermostate set at around 70 F, which would be around 21 C. That way, she will only need a light blanket and it won't matter if she does kick it off.
I hope that helps! Good luck!