Investing or paying a portion of mortgage off?

[Byff]

When me and the missus bought our first house, we had two endowments as was all the rage those days.

When we moved to a larger house, we took out a larger endowment, which covered the difference.

When the whole endowment's are crap erupted, it was around the time we put an extension on the house, so we switched to a part-repayment, part-endowment mortgage of 50/50 proportions to cover any shortfalls to the value of around £100k.

Our first two endowments have matured so we're sitting with nearly £20k in the bank.

The wife wants to pay off some of the mortgage, but as our repayments are so low, I'm thinking is there is a smarter way of doing this?

Could we invest the money in either:
.
a, Some form of bank/ISA/savings scheme until the third endowment matures
b, Use the cash for a deposit on a BTL property
c, Put it all on red at the casino
d, Just do what the wife suggests and reduce our debt.

[Byff]

I forgot the option of blowing it all on coke and hookers, but I'm happy with asprin and babestation these days.

I know that paying it off is the sensible answer, but I'm still only 45 with the remainder of the mortgage due in about three'ish years. I'm no financial wizzkid (hence why I run a Cerbera as a daily) so I don't know if there's any super duper investments out there.

My pension is sorted (LGPS) and my mortgage repayments are easily manageable. I've already got one buy-to-let, bit of a disaster but it pays for itself and I've learned a lot in the process.

I'll probably just pay it off.