In an unfair dismissal case, the need for an employer to carry out a reasonable investigation does not require him to gather all available evidence. This follows from the fact that the burden of proof is neutral under Employment Rights Act 1996 s.98(4) - it is only at the initial stage under s.98(1) that the burden of proof rests on the employer.
Mr Crabtree was a staff nurse employed by the Sheffield NHS Trust from August 2000 until his dismissal in January 2008.