After 4 engineer visits and everyone saying that at 1300m from the cabinet 12mb was going to be the best speed I could get I have found a way to nearly double my speed. Dumped the homehub5 and replaced it with a Billion 8800NL and now have a steady 21.5mb and not one dropped connection.
So if you are on a longer line its worth a try for £68 from Amazon.
Thought I would share this as I found it so frustrating that I could get nowhere near the speed I was sold now im just 1mb away from the lower estimate so I can live with that.
That's an amazing speed for your attenuation 🙂