At your current rate of 5.15%, you're paying €1502 per month in interest on €350k, excluding relief.
Add €13k to your principal, and at 3.4% variable, you'll be paying €1028 per month in interest..
So you'll save approx. €470 per month in interest.
Over the 20 payments left, that's €9400 saved in interest.
Doesn't look good, does it?
But don't fret. Because you'll be saving the guts of €500 a month in interest, you could increase your capital repayment by say €250 a month.
So you'll be saving €250 a month on what you're currently paying, and you'll be paying your principal off at an extra €250 per month.
I'd certainly change, but that's just me.
P.S. All the above is rough figures. You'd have to work out all your exact details yourself.