OP, it really is the matter of whether you have enough patience to wait for the issue to be fixed (which I know is appalling as you'd absolutely expect to get what you pay for but it is what it is) or call it quits and move over to another provider.
It took a year and five months for me to get my speeds back from over utilisation!
Really sorry to hear you're experiencing speed issues.
I've located your account and I can see an issue with the network in your area affecting speeds at peak times - Ref.(F004091621). Our technicians are working to get this resolved and have set a review date of 26th April.
I'm really sorry for any inconvenience this may cause.
All the best
Tech fan? Have you read our Digital life blog yet? Check it out