Difficult if not impossible to say without some context. For example, if your mortgage balanace is very low, you have good income(s) and otherwise are in a good place financially, it may make sense to think about savings or pension planning/enhancements. Or if mortgage has you under pressure perhaps paying a lump off the mortgage is a better option.
Also will you need access to the money at siome point, as that will be an important consideration in terms of your decision to lock it away.
As a general rule I would say pay down debt, and if its not a tracker rate, then its probably more expensive debt than the return you might obtain, without taking unnecessary risk. But really depends on your circumstances, including financial.