Hi
The T&C's for the October 2009 discount tariff don't mention anything about a charge to leave the tariff, whereas on the December 2009 it says..
"If you cancel your Fixed Price Energy December 2009 supply
contract either by switching to another gas and/or electricity
supplier or requesting to change your gas and/or electricity to
another of our tariff(s) before 30th November 2009, we may
apply a cancellation charge (inclusive of VAT). The cancellation
charges are £30 for electricity and £20 for gas. This charge will
not be applied if the contract is cancelled due to a home move."
This leads me to believe there is no charge as they have clearly stated it on the other tariff, and are hardly likely to forget to do it on the other if one did indeed apply. I would however ask them about it before you do anything, just to be sure.