Hi Shawnee32,
From my information, ScottishPower are offering their customers on this particular fixed rate deal, which prevented you from having the massive increases in September 2008, a new capped rate, capping your prices, at today's rates, until September 2010. From their website, this isnt available to new or existing customers, except if you are maturing on your fixed rate deal. It also cap's prices longer than any of their other current fixed/capped rates...
Normal Capped rates, as with ScottishPower's normally come at a premium, albeit, a small one, however, I understand your's is free. Capped rates offer some real good security against price rises, but if prices drop, you also get these, something you dont get on a fixed rate.
Alternatively from checking their website, ScottishPower offer one of their very competitive online energy savers again, Online Energy Saver 6. This offers big discounts against their standard rates, (which is what your capped against) but obviously it isnt fixed/capped therefore your prices will increase/decrease as necessary compared to their standard rates.
Hope this helps