It is free, your bill wont go up from it, although there is a price rise later this year across the board, so read into that as you wish.
There was something i read before the upgrade online that told me it was free for the upgrade can't find it now though.
Edit - found this in the email for it.
Quote:
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We'll email you in the next 48 hours to let you know the exact cost of your order, even if there?s no change to your monthly bill. Don't worry, if there is a change to your bill, we?ll make sure you get the very best price we can give you based on the mix of services in your package. If you?ve opted in for a free speed upgrade, then don?t worry it?s free of charge and won?t change your monthly bill.
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