(Balance of probabilities, yes – IANAL)

This came up as a case study on some employment law thing I went on a while ago. Nothing that came up on a quick search rang any bells, but this seems to be the case that is commonly cited: Monie -v- Coral Racing Ltd

“The employee appealed. He had been dismissed. The employer knew that there had been thefts but could not identify which of two employees was responsible. It dismissed them both. The claimant had been one of the two and now appealed against rejection of his claim for unfair dismissal.
Held: The appeal failed. The employer had acted properly.”

Similar, which goes into more detail about the steps an employer needs to take (but I can’t find a free write-up):

Parr v Whitbread plc t/a Threshers Wine Merchants 1990[i]“A shop safe was emptied of cash during a night shift. Both the police and Whitbread carried out investigations. They discovered that the robbery had been carried out by staff, but the culprits could not be identified. However, the list of suspects was narrowed down to four employees and all were dismissed.

Claim. One of them, Mr Parr, mounted an unfair dismissal claim. The Employment Appeal Tribunal held that a group dismissal can be potentially fair, provided certain criteria are met.”[/i]

It may be difficult to be seen to be reasonable by sacking everyone, but everyone can be disciplined (presumably to the point of dismissal if sufficiently serious):

http://www.thisisoxfordshire.co.uk/news/4663050.Council_probe_fails_to_find_missing_cash/

“Police were called in after allegations £6,097.85 was stolen from a cash box at the Vale of White Horse District Council, in Abbey House, Abingdon, on July 31 [2009].

Officers advised the council to carry out an internal investigation which has been unable to establish if the missing money was stolen.

The case was shelved by police on August 28 because of insufficient evidence […]

All 15 staff at the Abingdon Local Services Point, which includes the cash office, have been disciplined but no one has been sacked.

Steve Bishop, Strategic Director and chief finance officer for the Vale, said: […] “Whether it was theft or not the fact remains there was an opportunity for someone to steal the cash and this is unacceptable, which is why all the staff in the Abingdon Local Services Point have been disciplined.”

This is not my job, IANAL, and the two legal cases above may have been superseded, but at one time they were “the law” AFAIK.