Why is my broadband connection so slow at certain times of the day?

I live in a small town in west wales , united kingdom, every day without fail at 4pm the broadband speed of my computer connection goes to well below dialup levels, and this lasts until well after midnight. I am told this happens to everyone in the area. Firstly , how is this done by the company, secondly why do they do it and thirdly is there anything i can do about this legally or any other way because it is an intolerable situation in the year 2010 to have a worse connection than when i first used the internet in 1997.