they say several thin layers are better than one thick one sometimes as the warm air gets trapped between the layers so if you have two 13.5 tog duvets then that should be ok, but I would then put a blanket on top of these to hold in the heat. We've got 2 oil-filled radiators in our garage and they're brilliant because once they get hot they hold the heat for a long time, they can be expensive to buy but they're energy efficient so you might find what you spend out on buying them, you save on energy costs in the long run as I imagine it's quite expensive to run halogen heaters!
I also agree about the bed socks thing. i'm sleeping in my matalan bed socks at the moment and they're so much warmer than normal socks.
We have an electric blanket too but no matter how safe they say they are I would not personally sleep with one plugged in. We use ours to heat up the bed an hour before we're due to get in it and then unplug it just before getting in for the night, they warm it up nicely for when you get in and with the other ideas from everyone you should be nice and warm and toastie in no time.
I find if I get into bed and i'm already cold and the bed is cold then it takes me a long while to get warm and i just cant get off to sleep, but if i use these little tricks to make the bed warm first and get me as warm as possible for when I get in then I stay warm once i'm in there.
I do hope you manage to find something that works for you really soon as it's only going to get colder from here on in!
good luck with this, keep us posted on how you're getting on.