A VAT Assessment Letter is sent to you if you do not submit a VAT return for a particular quarter and pay the VAT due on time. HMRC issues this letter and it contains an estimate of how much VAT think believe you owe them and demand the payment accordingly.
What to do about a VAT Assessment
You must submit your VAT return payment immediately, along with submitting the missing VAT return within 30 days.
Failure or late submission your VAT return and payment can result in surcharges (a type of penalty) being charged to you.