I don't think I would except that...Because when you did a linespeed test it's not testing your limewire download speed, it's testing your overall speed and your ISP is responsible for that...
I would ring them back and strongly
suggest that they supply the speed you are paying for in your contract or offer you a discount or let you out of your contract so you can go with an ISP that will deliver what you pay for...
Stand up to them...