SolvedCH63160 | Penalties for failure to file on time: Penalty deductions for disclosure: Determining unprompted or prompted disclosure
From HM Revenue & Customs · Compliance Handbook
Whether a disclosure is unprompted or prompted is an objective test. It is not what the person believed but what the particular facts and circumstances gave the person reason to believe, taking into account their circumstances and abilities.
A national campaign highlighting an activity or liability on which HMRC will be concentrating does not stop a disclosure from being unprompted.
However a disclosure is prompted if a person makes the disclosure after
they become aware that we have obtained, or were about to obtain, information about something that they have withheld by not filing the return, or
we have contacted them regarding the particular tax or activity to which the obligation to file relates, or
during the course of a compliance check concerning one tax liability, it becomes apparent that they have failed to file a return for another tax or duty.
For practical examples of unprompted or prompted disclosure, see CH63180.