As dinger said contact the OIC (Officer in Charge)
Also at court there will be a Witness Liasion Officer. This is a voluntary person, often a kind old person who alleviates fears.
A few further points. If you withdraw, there may (and I stress may) be a chance you are summonsed yourself to testify. Known as a hostile witness, you don't really want to go down these lines. Any refusal to attend can lead to YOUR arrest and at court, contempt charges may be brought against you. This is regardless of the actual crime you are there for!!!
Secondly, if you are scared of the assailants. It must be remembered that if any funny business does occur, witness intimidation is probably a worse offence than what you are testifying against. The defendants will know this.
And finally, rest assured YOU ARE NOT THE ONLY WITNESS. There will be police witnesses and more than likely a doctor (if there were any injuries). Although they are treated slightly different to yourself at court, the rudiments of their testomony is exactly the same as yours.
Good luck. Honestly it is not that bad. Just take your time, address the Judge and not the counsel and chin up.