Yes, it will come off the balance and reduce the term.
Some lenders charge the interest in different ways, so it's worth checking with your lender if they will recalculate your interest when you make the payment.
I'm guessing here, but are you with Nationwide? If so, I think they do recalculate the interest from the day after you make the additional payment. This the usual way, but some wait until the end of the year until they recalculate. Having said that, even if they wait, it's probably still best to pay in the £300 pm since there is no other way of paying off the balance as a lump sum without paying a much worse penalty.
Paying an extra £300 pm will reduce your remaining term to somewhere around 10 years. You may find you could increase the level of overpayment over time - if so, just check what your lender will say if you clear the mortgage completely if you are still in the fixed rate - Northern Rock for instance won't charge the penalties, unless you clear the mortgage in full - that last £1 would be expensive.
As a top tip - you may find it advantageous to leave yourself with a mortgage of a £1000 or 2. It's not expensive (£5 - 10pm), but should you ever want to borrow money on the house in the future it is sometimes easier/cheaper if you have an existing mortgage.
Good luck, I hope it works for you.
Adrian