From this website it looks as if the law has been changed.However,it only covers sperm donors from the date of the law change FORWARDS(2005) not for any donation of sperm backwards from then.
In other words,any children conceived in this way(from 2005 onwards) WILL be able to trace their Father,sadly those before 2005 will NOT.
As you are already conceived you fall into the latter category.This is what I understand from this article,but have a look and see what you think?
http://news.bbc.co.uk/1/hi/health/4397249.stm
Here are the Google search results for this,which may give you more information:~
http://www.google.co.uk/search?hl=en&q=sperm+d onors%2Banonymity&btnG=Google+Search&meta=cr%3 DcountryUK%7CcountryGB