It's no different to simply over-paying your mortgage. I've said it before and it's not really rocket science.

Very roughly......

You mortgage payment of, say, £800 is made up of probably only £200-£250 capital, the rest is interest. So, if you can overpay your mortgage by £200 per month, then you will cut your term significantly, saving you £500-600 worth of interest for EVERY £200 you overpay. Thats the rough idea.

As far as I know, the One Account is only beneficial if you have money left each month, which if you do, you might aswell use to overpay a conventional mortgage, provdinig your lender will let you (most do).