Should we pay lump sum off mortgage/reduce term

Thanks for that.

On the subject of the CALCULATOR, whereabouts would you put in a one-off payment?

At the bottom of the screen, there's a button called Extra Payments. Click on that. For Extra Payment #1 enter the amount you will be paying off. For Start Month # enter the month you would be paying this off, ie, if you're paying it off in March, enter 2. For End Month # do the same, enter the same number you had for Start Month #. Otherwise it will assume you're making the same extra payment every month.
 
I get what you're saying about the psychological side of things and paying a chunk off the mortgage in order to make it more pleasing to the eye. I was having a similar vein of thought recently about my own mortgage but when I crunched the numbers, it would have been financial suicide to pay off early since I am getting 21% net from An Post over 5.5 years and my SVR mortgage is just 2.85% APR. When you see the tens of thousands involved, it makes you realise that in some circumstances, paying the mortgage off early is foolish