F****** ****ed Off

Been struggling for over 2 weeks now with poor download speed. Supposed to have at least 6Mbs but only on average have a third of that. The speed is all over the place. they just keep telling me that from their side the speed is ok ??? But when do the test they recommend its nowhere near the speed I'm supposed to have. Was working fine until about two weeks ago so not sure what's changed? I'm seriously disillusioned with them. They've been fairly good in the past?