VATREG41400 - Registration in respect of distance selling from other Member States: failure to notify
The Commissioners have a legal requirement to register traders from the correct EDR regardless of their failure to notify their liability, or their failure to do so at the correct time. If you discover such a case, you will need to calculate the amount of tax due from the date the person became liable until the date we received notification or became fully aware of the liability to register. Any VAT already paid in another Member State should be deducted from this amount.
Once you have established the net tax arrears, you may wish to go on to consider whether a civil penalty is appropriate, see VATREG26000.