If you are required to go to court you must do so. If you do not turn up, the court can issue you with a witness summons. This means that you will receive an official document requiring you to attend court at a certain date and time. If you ignore the summons, a warrant can be issued for your arrest. If you are worried about giving evidence, tell the police (or the defence solicitor if you are a defence witness) or the Witness Service so that they can offer you support and help.
that's from this site
http://www.cjsonline.gov.uk/witness/walkthroug h/going_to_court/faq/