I had the same problems with Zyxel router due to my long line length, sounds like you are as well. I phoned and got through to 2nd line technical support (well actually, these guys called me) and they sent me a technicolour router. I switched it out, speeds more than doubled from 1.5mbps to 4 (which was more that was Bt wholesale said the line could manage)
It might be worth trying to get hold of their technical team and see if you can have the router. If they don't budge you can have mine if you like!
BT ADSL 4.7Mbps (Actual)