It sounds like unfortunately that you are being asked to attend a disciplinary hearing. Have you done anything wrong that you are aware of? Your employer cannot give you any official warning whether it be verbal or written without you attending a disciplinary hearing. You must be given at least 24 hours written notice and you must be offered the chance to take in a colleague or union official with you. I would have thought that the letter you received, would have been clearer though and given the reasons as to why they want to see you.
Disciplinary hearings can result in dismissal, but normally this is only when you have committed gross misconduct in which case they are entitled to sack you without notice pay. I am certain that you would be aware if you have or have not committed gross misconduct, so I am guessing it is for something less serious, like perhaps a falling off in the quality of your work? If you are a union member, I would seek further advice from them. If your company is unwilling to discuss this any further with you before the hearing, I would contact an advisory service for further advice as you need to ensure that you are aware of all your rights before attending the hearing. Best of luck, I hope this has not worried you too much.
Let us know how you get on.
Sue