The law didn't change. The Crown Prosecution Service has aways been able to present any relevant evidence (including statements which have subsequently been withdrawn) before the courts.
It's
attitudes and
policies which have changed over the years. There was a time, within the memory of many people, when the police would hardly ever intervene in a domestic dispute involving violence. The attitude was that what went on 'behind closed doors', involving adults in a domestic setting, was not the affair of the courts or of the public. As public attitudes have changed, the policies of the police and CPS have gradually caught up with them.
The CPS were seeking to improve conviction rates, through using 'withdrawn' statements in certain cases, throughout the latter part of the last decade and the first part of this one. The current policy was then set down in black and white in February 2005:
http://www.cps.gov.uk/publications/docs/Domest icViolencePolicy.pdf
Chris