Paying off loan v overpaying mortgage

Arbie

Evening all,

First, the facts:

Mortgage @ 2.5%, 21 years to run. £851/month

New Zopa Loan of £8300 @ 4.7%, 3 years to run. Repayment £250/month

And the question:

I am in the fortunate position of being able to overpay my mortgage each month by £850. The basic rules of paying off the most expensive debt first would suggest that I should pay that money to the Zopa loan (ie a total of £1100/month until it's cleared; approx 8 months), and then overpay the mortgage by £1100/month for the remainder of the three years I would have had the loan for.

However, as my mortgage overpayment reduces my capital this saves me interest year on year; is there an easy way to work out how much this is actually worth and what point it is preferable to keep overpaying the mortgage and pay off the loan concurrently?

I realise that with the interest rates as they are it probably makes it fairly clear cut to pay off the loan first, but I'm interested in the maths so I can apply it in future if necessary.

Thanks from a relatively numerate, but foxed, moneysaver!