Readers’ forum: What is the basis of the legal obligation to disclose errors?

I’m confused about the legal obligation to disclose past errors and I am hoping that readers can point me in the right direction.

FA 2017 introduced legislation requiring a person to correct past non-compliance in relation to offshore matters or face a penalty. Parliament introduced legislation to change the law so it would seem that before FA 2017 there was no general obligation to correct non-compliance and that now there is only an obligation to correct non-compliance in relation to offshore matters.

Under self assessment there is in any case no mechanism to adjust a return after the enquiry time limit has expired. Yet the professional bodies have always made it clear (see PCRT help sheet 3) that there is an obligation to disclose past errors to HMRC. What is the basis for the advice in PCRT?  I should add that I completely endorse the...