Hi charliekitchenham,
As Raunchz said, i wouldn't recommend driving around with only three nuts, for a start your car would be classed as 'unroad-worthy' as you can fail an MOT if you do not have the correct amount of nuts in each wheel. I'm not sure what the legal implications would be but i would imagine a small fine, possibly points (depending on coppers mood).
What you need (as said above) is an extractor kit. If you pop into your local halfords/motorshop and explain to them the situation they'll be able to find you the kit you need. Its around £6.00/7.00.
The kit will contain a few different sized small bars that have a reverse thread on. You drill a small hole (just a little smaller than the bit that you're going to use) in the centre of the snapped nut. You then screw the extractor-bit into the hole.. as you would screw a screw into a piece of wood. Once the extractor bit gets so far it will start turning the snapped-nut the opposite direction due to the extractor-bit having opposite direction thread on.
I'm not sure how well i've explained that so i've gone one step further to help you and got you some pictures
Here's a link to the type of kit you need:
http://www.halfords.com/webapp/wcs/s...egoryId_165572
If you need any more help just drop me a pm mate.