Are you sure that it's 12Mb. That may be the service that your signed up for with your ISP, but unless your on cable, it all depends on your BT line. Try this: http://www.speedtester.bt.com/ . The package I have with my ISP is up to 8Mb but because of the BT line bottleneck, I generally get between 3.5Mb and 3.75Mb.
Iain.