In reply to XXXX:
£10k at 4.7% over 20* years is £64 pcm.
£10k at 3.6% over 3 years is £293 pcm.
If you put the £10k into a bank account and pay £500 pcm off the mortgage and £293 pcm for the loan the £10k will run out after 12 or 13 months (£10k divided by £793pcm). If you get interest on the £10k tapering down to nothing over those 12 or 13 months it might mean the money runs out a few weeks later.
You now save £64pcm on the mortgage but must pay another 24 months at £293pcm on the loan. OUCH!!!!
I don't think you can pay off some of the mortgage quicker (3 years) without it costing more over those 3 years.
*a guess as I don't know how long your mortgage has to run
Repayment loan figures (arrangement fee option set to "No") from
http://www.moneysavingexpert.com/mortgages/mortgage-rate-calculator
Edit - as Minneconjou Sioux says - amortization.
Post edited at 18:25