EM5170 | Penalties: Culpability - Defences: Failure to Make a Return
From HM Revenue & Customs · Enquiry Manual
A customer may claim that they did not make the return on time because
their affairs were too complicated to be able to complete the return in time
they were let down by their accountant, in whom they had implicit trust
of illness, absence or family problems
they were too busy running the business
the records were lost preventing calculation of the correct figures
they had completed the form as far as possible and that the rest of the information would follow as soon as they had time or could obtain the required particulars
the delays were shorter than those of HMRC
the missing information had already been supplied in some other context.
See CH160000 for further guidance on what may be deemed as ‘reasonable excuse’.