The invention relates to a fungicidally active compound of formula (I): where Het is a 5- or 6-membered heterocyclic ring containing one to three heteroatoms, each independently selected from oxygen, nitrogen and sulphur, provided that the ring is not 1,2,3-triazole, the ring being substituted by groups R8, R9 and R10; X is a single or double bond; Y is O, S, N(R11) or (CR12R13)(CR14R15)m(CR16R17)n; m is 0 or 1; n is 0 or 1; and R1 to R17 each, independently, have a range of values; to the preparation of these compounds, to novel intermediates used in the preparation of these compounds, to agrochemical compositions which comprise at least one of the novel compounds as active ingredient, to the preparation of the compositions mentioned and to the use of the active ingredients or compositions in agriculture or horticulture for controlling or preventing infestation of plants by phytopathogenic microorganisms, preferably fungi.