SolvedCH26710 | Information & Inspection Powers: Penalties: What is the penalty: Failure, obstruction, concealment etc: Amount of increased daily penalty
From HM Revenue & Customs · Compliance Handbook
The maximum amount of increased daily penalty that the tribunal can approve for failing to comply with an identity unknown notice, is £1,000 per day.
When the tribunal decides on the amount of the increased daily penalty, it must consider
the likely cost to the person of complying with the notice,
any benefits to the person of not complying with it, and
any benefits to anyone else resulting from the person’s non-compliance.