Investigate thoroughly. If you take disciplinary action and are sued for unfair dismissal as a consequence, you have a defence if you can prove that you investigated thoroughly, and acted in a reasonable belief of guilt on the basis of that investigation. You will not be expected to prove the employee's guilt beyond reasonable doubt.
If you are right, the employee is unlikely to go to an Employment Tribunal; but if you are wrong and he (or she) does, the Tribunal is most unlikely to find in your favour if you have sacked someone on a mere presumption of guilt. If a criminal offence is involved (for example, theft) consider calling in the police. Take legal advice.
