There is a rebuttable presumption that the defence will have presumably rebutted (don't think thats a real word but you understand the gist) under SOA 2003 (75)(2)(f)
(f) any person had administered to or caused to be taken by the complainant, without the complainant's consent, a substance which, having regard to when it was administered or taken, was capable of causing or enabling the complainant to be stupefied or overpowered at the time of the relevant act.
In this instance, the girl was presumably drunk on her own free choice, the accused did not administer alcohol to the victim without her consent. The defence presumably succesfully argued that defendant reasonably beleived that the girl "agreed by choice, and had the freedom and capacity to make that choice"(74).
The issue of wether she had the "capacity" to make this choice is debatable, the defendant may not have been aware that she was drunk (unlikely I know, but I for one would not turn down sex with a girl who was offering it to me whilst slightly inebriated)
That wasnt a very well constructed arguement I know, my point is that as the law currently stands the decision made by the court was correct. The law on rape has also been changed relatively frequently (2003), so we are not likely to see an immediate change in the law.