What Makes A Disciplinary Investigation Reasonable?

This article was first published in People Management on 18 October 2011:
http://www.peoplemanagement.co.uk/pm/articles/2011/10/what-makes-a-disciplinary-investigation-reasonable.htm

The law doesn't set out clear rules on how to carry out disciplinary investigations, but employers still need to take steps to act fairly and impartially

There is no right or wrong way of conducting disciplinary investigations. Provided there are no contractual requirements or agreements with trade unions or other employee representatives as to how investigations need to be carried out, it's a question of whether the employer's handling of the situation was reasonable or not. But what does an investigation need to include to be seen as reasonable?