Get a copy of the company policy for such hearings.
You should be entitled to have a witness with you, for the hearing.
If you belong to a union you can take the union rep along.
Otherwise take a very competnet and cool friend or even a solicitor.
Write down everything said at the meeting.
Don't agree to anything at the meeting either verbally or on paper.
Do not get angry or upset at the meeting.
Ask for specific examples of everything that comes up. If they disabled your email and blackberry so you cannot ansewr the questions, make this quite clear and do not agree to anything they suggest.
You must insist on getting at least archived copies of your mail and calendar. Although the information may belong to the company you are entitled to view it if it is being used as evidence.
If they prove intractable at the meeting say you cannot answer certain points until you get legal advice.
I would advise ringing ACAS or a solicitor dealing in employment law.
You also need to raise the issues with HR for the areas where you have been asked to supply information but have been denied access to your calendar so cannot confirm.
write it all down and maintain a full paper trail.