 During the 1960s and 1970s, he assembled one of the best collections in private hands, in spite of competition from Paul Mellon among others.During this period, Constable scholars began to distinguish more rigorously between the works of John Constable, his son Lionel, and followers.In the early 1980s, Manton came to know Leslie Parris, deputy keeper of the British Collection at the Tate, who, together with Ian Fleming-Williams and Graham Reynolds, were the leading authorities in the field.