First, a quick history lesson. The Public Interest Disclosure Act 1998 (my emphasis) created protection for whistleblowers from detrimental treatment but, despite its name, there was no need for a qualifying disclosure to be in the public interest.
As a consequence, it was not too long after its introduction that the Employment Appeal Tribunal ruled that a worker could make a disclosure about a breach of his/her own contract and that that could amount to a qualifying disclosure (Parkins v Sodexho  IRLR 109).
In 2013, the then government sought to change that, by introducing a public interest test into the legislation, such that a worker would only be protected if he/she had a reasonable belief that the disclosure was in the public interest.
That of course created scope for case-law as to what being “in the public interest” means in practice. My colleague Kathryn Pickard reported here on the judgement in Chesterton Global Limited v Nurmohamed (UKEAT/0335/14), in which the EAT considered the meaning of “public interest” and concluded that it is not necessary to show reasonable belief that a disclosure was of interest to the public as a whole, and that interest to a relatively small group of the public could be sufficient to satisfy the test.
And now we seem to have come full circle, or almost full circle. The EAT in a recent judgement (Underwood v Wincanton plc UKEAT/0163/15) has ruled that a worker who made a complaint about a breach of his/her terms could be protected by whistleblowing legislation. In other words, it is possible for a complaint about a breach of an employment contract to amount to a qualifying disclosure, even with the new[ish] public interest test.
In Underwood v Wincanton, a group of employees (four HGV drivers) complained about the way overtime was allocated among drivers. Mr Underwood, one of those employees, made reference in response to a Tribunal order to the fact that some of the drivers who were granted less overtime had raised concerns regarding the safety and road-worthiness of vehicles. The Tribunal ruled that a dispute between employer and employee about contractual terms is not something which the public are affected by and struck out the claim. The EAT disagreed. Those making the disclosure in this case had been raising concerns of vehicle safety and road-worthiness, and that could be thought of as a matter of interest to the public or a section of it. The claim was therefore allowed to proceed for hearing by the Tribunal.