I was employed for 3 months. But as you know length of employment does not apply to whistleblowing cases. And my ET has been accepted.
Can you please clarify on what grounds do you believe the protected disclosure was made. The accepted grounds are here: https://www.gov.uk/whistleblowing/overview
Systematic underreporting of hours worked translates directly into underreporting of taxes due to the HMRC, and this is reportable to the HMRC, and therefore a disclosure in the public interest.