I make it 10 years @7% compound (monthly), to double, fwiw.
@twoboys,
I imagine the appointment with the provider will revolve around what the early payment penalty fee will be, how that varies with time (relative to the originally scheduled end date) and negotiating the revised end date and payment schedule with you. I'll take i as read that you're already familiar with what the 'administration fee' will be, over and above any penalty. (If not, it'll be in the leaflets sent with the annual statement).
One other factor to consider, which hadn't occured to me until now. If the property is likely to make her estate stray into Inheritence Tax territory, then I see every reason to leave the mortgage in place. She may well live long enough to pay it off in full, wherupon she could take out another one (unless the InhTax threshold has been revised upwards in a game-changing manner, by then). If not, the deduction off her estate by the mortgage provider will reduce any Inh tax payable.
Or, put another way, you'll need to discuss all this with an independant financial adviser and/or qualified accountant. I am neither, obviously. :-/