I think my account has a 500 overdraft limit and i haven't been charged before for going into the minus.
Failing all that you will get a 30 quid charge somewhere along the line.
I think Barclays is a thieving bunch of so and sos and thats just my opinion (and now looking to shift banks as I am chuffed off with them, I have a couple of good stories about them and their cockups )