Sorry but WHAT? The guy assulted you publically, regardless of 'medical evidence'!! Sue the police!
TO the OP, sorry to hear about your assault. Hope everything goes ok with the treatment.
To Nero120, bailing them for medical evidence is standard practice. As someone else has stated you can only someone for 24 hours (for normal offences) before you either have to charge them, and either bail them to appear at court at a later date / keep them in custody and put them before the next available court if the matter is serious (with a view to getting them remanded in custody) OR you bail them for further enquiries.
There are probably quite a few statements to take from witnesses as well, so this will give them time to do it.
The CID Officers dealing with the case are Police Officers not Doctors. For the sake of a far stronger case they have bailed the Offender to seek expert medical opinion and obtain statements from them.
The long term physical problems are not known. Hopefully the OP will make a full recovery, but there could be nerve damage, blurred vision and countless other problems associated with a serious head injury.
The physical injuries are often just part of the problem too, since many victims of assault are traumatised, they shun the places they used to go to, their friends, their family and it can often devastate their lives. It can take far longer to heal the mental scars than the physical ones.
If the OP is still reading this please take advantage of the Victim Support Service that I hope you have been offered - they're excellent people and often have been victims of similar sorts of crimes - they can help.
Secondly ensure that at some point you make enquiries to obtain a CICA claim form. An article as to what they do can be found here:-
http://en.wikipedia.org/wiki/Criminal_Injuries_Compensation_Authority
Their official website is here:-
http://www.cica.gov.uk/
You can apply straight away, but they regularly do not settle until the case has been dealt with at Court (since they need to know what charges have been brought and the results).