I feel for you mate - really do, since March this year my otherwise rock solid 4Mb connection has dropped right down to 160Kb, practically useless for working from home.
I've contacted Demon a number of times and managed to get 3 engineers out, all completely useless neither aware of the previous engineers notes due to BT openreaches (lack of) CRM system, so every visit starts right at the beginning - then after 2 hours the engineers are gone in a puff of smoke, cause they only get paid for 2 hours.
At the point now where the last engineer told Demon that I was unable to get Broadband - which is bullsh1t because I had it for 4 years at 4Mb.
I'm at the point now where my options are, move house or change job.
Hope you have better luck than me.
Dave