At present mesnplat you can only be accompanied by a trade union official or work colleague at a meeting and many work colleagues do not want to put their head over the parapet, your employers may let you be accompanied by another person but they do not have to.
Hopefully your employer has suspended you on full pay, they should have done. At the meeting explain your position and explain it was a mix up not the intention to deprive your employers of money, submit written evidence from the person whom you repaid. Your contract of employment will probably show theft as gross misconduct and they may, if they do not believe you, sack you with the right of appeal, you have to be notified in writing of this. You do not say how long you have worked for this employer but if it is at least one year you will be able to claim unfair dismissal at an Employment tribunal, watch the time limits on this.
My experience is that if you are a valuable member of staff they will not attempt to sack you but you do need to be persuasive at the meeting.