The Commissioners may, subject to such conditions as they see fit to impose, permit the mixing in an excise warehouse with made-wine (whether imported into or produced in the United Kingdom F1or removed to the United Kingdom from the Isle of Man) of duty-free spirits in a proportion not exceeding F212 litres of alchohol to 1 hectolitre of made-wine, so, however, that the mixture shall not by virtue of this section be raised to a greater strength than F218·3 per cent.