(c)the use of a certification trade mark in relation to goods or services adapted to form part of, or to be accessory to, other goods in relation to which the mark has been used without infringement of the right given as aforesaid or might for the time being be so used, if the use of the mark is reasonably necessary in order to indicate that the goods or services as so adapted and neither the purpose nor the effect of the use of the mark is to indicate otherwise than in accordance with the fact that the goods or services are certified by the proprietor.