You appear to be capped at 1024, probably due to faults in your wiring. Read this article and take your stats from the Test socket, then post them here.
Sky User - The unofficial support forum for everything Sky! How to test your Master Socket
One thing you should understand, you are using an 'Up to 16MB' package, but with your distance from the exchange, you will never get anything like that. If your external line is very good you may get around 3-3.5MB.
PS I have just noticed you have posted the same thing in another part of the forum, you should really not do that as it leads to cross and duplicate posts.