DD aged 2 is wriggling out of car seat straps - help

[jujubean]

We have a group 1-2-3 car seat for DD 2. She has started twisting and wriggling to get her arms out of the straps so that all is left holding her in is the little bit around her hips clipping into the main clip bit at the front. Not good at all. She twisted out on Saturday on a long trip to my parents so today on the way back I pulled the straps as tight as they would go. She kept wriggling out and we kept stopping at the services to pull her in tighter. It seems she can get out no matter how tight the straps and obviously the car seat is pointless if she's not held in it. Discipline doesn't work as she can't go on the naughty mat on the M40.
Any ideas? I'm willing to buy a new car seat if neccessary but are there any gadgets out there to help with this?

[nicm]

the in car safety centre sell chest clips. www.incarsafetycentre.co.uk/shop/accessories.html

they are a bit expensive at 17.00 though. does she have her coat on? it's harder for them to get out i find when they don't have a coat on.