Might be the cheapest money alright but the savings may be eroded if their are legal fees, extra life assurance and a house valuation required should the op decide to go the mortgage top up route. In terms of time the op will be doing very well to sort out a top up between now and the end of January imo - a variable rate term loan might be the only practical solution for the op whereby he can clear the outstanding balance , without penalty, once his ssia matures.