The fact they have taken four months to deal with this suggests to me that their case may be flawed. You are entitled to have had this matter dealt with promptly, and to know in advance the precise nature of the allegations against you. Should they dismiss/discipline you, and you have grounds to take them to an industrial tribunal, their failure to follow correct procedures or respect your human rights may count against them.
You should also have been suspended on full pay - if not, you must immediately seek legal advice as you may well have a case against them for constructive dismissal.
What will happen at the hearing depends on your employer. Essentially they will have some kind of disciplinary procedure which should be explained in your contract of employment or be part of your implied conditions of employment - for example, displayed on a noticeboard for all to see or made known to you via a memo, e-mail etc.
Whatever form the hearing takes, it is NOT a court of law and its decision is not final. You have the right as others have said to be represented, by a union or professional body or by anyone else of your choice.
I would suggest you contact Citizens Advice for an appointment in advance of your hearing. They may agree to write to your employer and/or attend the hearing with you, or they may suggest that you seek legal advice and representation. If that happens, you're entitled to ask for the hearing to be deferred so that you can prepare your case and be properly represented.
Good luck!