In UK law for Civil Cases, as this is, the burden of proof is on a balance of probabilities.
Appart from contracts for real property (Land and Buildings and Section 1 Consumer Credit Act Agreements) nothing has to be written.
The problem with contract law is whether what was said is deemd to be wither a "Term" of the contract or an Inducment to Contract. If it ain't a term the employees do not have a pot to pi
s in!
As there is no jury, if the judge there is a strong possibility that the Judge may fall on the side of well respected profs. Such as Airline Pilots. Then again he may not.
I have not checked for case law on this so it may have been decided in a previous case. (Most Likley)