This is a tragic incident where the victim has sustained injuries consistent with GBH, however, from the information disclosed in your postings I would agree with the initial comment from the police regarding the issue of intent to commit GBH and that recklessness is a more probable and accurate assessment of this event.
To assist you, the two main offences that cover unlawful assaults resulting in GBH injuries are, Sec 18, and Sec 20, Offences Against the Persons Act.
Sec 18, is the more serious offence and is whats known as an offence of 'specific intent. Sec 20 is an offence of 'basic intent'. The basic difficulty with the Sec 18 offence is proving that the accused intended to cause unlawful and serious injury. The Sec 20 offence can be committed recklessly and therefore self-intoxication is no defence.
From your description, the accused has not disputed his involvement and the fact that he has told police he can not remember the incident is not a major issue. I would suggest any issues would revolve around whether this is considered an unlawful assault and therefore whether or not a criminal prosecution is in the public's interest to pursue (this is the second test whether of a there is a realistic chance of a prosecution, the first being an evidential test).
I will post a few links of the offences for you to read.