A failure to complete or return a notice within 21 days is an offence, as is providing false or misleading information on the notice. You should immediately obtain professional advice.
The local council will only issue a Planning Contravention Notice when they believe a breach of planning control has taken place and they want to obtain further information before they decide whether (or if) to take planning enforcement action.
Issuing a Planning Contravention Notice is discretionary; a local council doesn’t have to issue one before taking enforcement action.
Consult the Government’s website for more information on planning contravention notices.