Some lenders will not allow you to do this or will charge you (eg- Bank of Scotland: however, we are looking into the legality of them making such a charge).
You need to read the terms of the agreement to find out if they will allow you to make increased payments, or alternatively, save the balance of the cash you have available & make an yearly contribution.
Bank of Scotland (as it stands currently), do allow us to make lump sum payments which will reduce the monthly payment, but NOT the term - adjusting the term carries a €95 fee each time. I'm not sure who you bank is so the T&C's could be different (EBS was our previous lender and they had no penalties for early payment & adjusting the term downwards)
Talk to your broker as he will know the bank & how likely they are to stand over what they have said.