Buy Additional Pension Years or Invest Elsewhere

I am looking at buying additional years for my public sector pension. I'm 51 now and can buy a max of 8 additional years up until age 65.

The cost of buying one additional year after tax relief is ~€1,200, with 14 years of payments until I am 65, so say €16,800 in total to buy one additional year. That will return me an additional gratuity of €4,900 and an increase in my pension of ~€100 per month. If I bought 4 additional years (probably the max I could afford right now) it would cost me €67,200, returning me an increased gratuity of €19,600 and an increase in my pension of ~€400 per month.

On the basis of the above it would take around 10 years for my investment to make a return over the total sum invested. At that stage I would be 75 if still alive! To my mind that doesn't make the investment an attractive option.

If I had €400 per month to invest over say the next 10 years what would be the best way to do this to get a better return? Would I be better using it to pay down my current mortgage at 3.8%? Should I just put it in savings?