B8: Errors in returns
No general duty to tell HMRC if subsequently realise return erroneous
“In contrast, there is no statutory provision imposing an obligation on a taxpayer to tell HMRC about something in a filed return that he subsequently finds to be erroneous. The most that can be said is that a failure to correct an error can potentially affect a taxpayer's exposure to penalties (see ss 95 and 97(1) of the TMA). There is thus no question of Upton Wilson having ignored a provision obliging a taxpayer to correct a return” (Sanderson v. HMRC  UKUT 623 (TCC), §36(1)).
No false statement if taxpayer’s uncertainty identified in return
“The declaration that the person signing the company tax return is required to give is that the return is correct and complete “to the best of [his]/[her] knowledge and belief”. That statement does not require the signatory to certify that the return is absolutely correct. It simply requires that person to complete the return with the best information that is available. If the position is uncertain, the person who completes the return can identify that uncertainty to HMRC and will not be regarded as making a false statement by doing so (see Goulding J in Dunk v General Commissioners for Havant and others  STC 460n).” (AEI Group Ltd v. HMRC  UKFTT 290 (TC), §74).
Failure to disclose error will mean it is treated as careless