Just found these, might help. They are from the CJS website here is a link FYI :o)
http://www.cjsonline.gov.uk/witness/walkthrough/going_to_court/faq/
What if there is a problem I can't sort out?
If there is a problem that you really cannot sort out and you are unable to attend, you need to contact the person who asked you to appear as soon as possible in advance of the court case. You should allow them enough time to make the necessary alternative arrangements.
In this situation if your evidence is not crucial to the case you may be excused from attending. If your evidence is essential to the case, the lawyer may ask the court for an adjournment. This means that the case will be heard at later date. This is difficult to do especially if the accused person is in custody.
You should remember that your attendance in court could make all the difference to the outcome of the case.
What do I do if the trial date is on a date that I cannot attend?
You should contact the person who asked you to attend court immediately so that the they can decide what needs to be done.
What happens if I cannot afford to go to court?
You are entitled to travel expenses. If you cannot afford to get to court and need the money in advance you should let the person who asked you to come to court know and they will try to sort something out.