Well one of my welsummer girls is determined to sit and the other's are laying on top of her

as it's the favourite space, so I just put some marked call duck eggs under, then I lift her up at the end of the day, take the fresh hen eggs and plonk her down again and she's still sitting quite happily on the call duck eggs
[size=78%]. [/size]Once the broody hormones have really kicked in it's hard to stop them.
You can take them and dunk them in a basin of cold water (don't submerge them obviously) and hold them there for a minute or two, you need to 'shock' them out of it and take them off heat. It does work but you might have to do it more than once. Sounds barbaric but Hugh FW did it once on River Cottage and it doesn't hurt them. It takes a lot out of them being broody and sitting so if you really don't want it then it's best to do it asap.