In reply to colina:
So he's committed to paying £34200 over the next 4 years and 4 months.
Invested at 3% (assume that's apr) for 4 years and 4 months:
End of year 1: £34200 x 3% = £34713
End of year 2: £34713 x 3% = £35233
End of year 3: £35233 x 3% = £35762
End of year 4: £35762 x 3% = £35941
End of year 4 + 4 months: £35941 x 3% = £36480
And then he uses £34200 to pay off the mortgage and he's left with £1767
If he simply uses the £34200 to pay off the mortgage now, the mortgage has gone and he has an extra ~£650/month. £650 x 52 = £33800 and that's assuming he puts it under the pillow and doesn't invest it. So, on teh face of it, unless there's an enormous penalty for early repayment he should pay off the mortgage.
However, I'm not sure that what you said in your original post can be true. If he were really paying ~£650/month interest only on £34200 then the apr would be huge! Of the order of 20%?