I need your help people!

In the new year I will be able to make overpayments on my Northern Rock 'together' mortgage. Northern Rock said I can make overpayments on either the secured bit or unsecured. Which should I do? I'm thinking the unsecured bit to get it down! Unfortunately I took out a 125% mortgage and have about 28k left unsecured to get rid of.

Thanks for your help!