Is she broody or is she just having me on?

I bought 2 Silkies to be broody hens early last Spring. So far they have proven to be complete duds at it, laying little cream eggs for all they are worth and generally being as un-broody as you can get! I eventually resorted to buying an incubator, which is currently full. Then, lo and behold, a week after I got the incubator one of the Silkies went broody. Or at least I thought she did. She was sitting on one of her own eggs, all fluffed up and spread out, and made little cheepy-growling noises when I lifted her up, and when I put her back down she went back to sitting in her nest. So I got her some lovely eggs. I put the eggs under her last night but this morning she was sitting beside them. I lifted her back onto them, but every time I go to check she is sitting beside them.  I've put her back on them 3 or 4 times now, and she keeps shuffling off them, and I'm not sure what to do.

Firstly, if she's sat on them overnight (since 11pm) but has got off them before 7am, will they be ruined? I have had a broody who sat in a cardboard box quite happily for the length of time it took to hatch her eggs, and I lifted her off for a few minutes twice a day - should I try putting her in a smaller nest area to keep her sat in the right place? If she won't sit on the eggs I will have to break her broodiness somehow, but don't know how to do this - I have always just popped a few eggs under broody hens in the past and let nature take it's course.