Well I've managed to solve my problem.
The male parent had 2 spouses - both with children. While adding census data to one of the children from the 2nd wife it managed to add a link from one of the children of the 1st wife to the 2nd wife. So this child appeared to have sprung from both wives! I deleted the child and that fixed the tree.
Fortunately the problem child has no descendants in the tree at the moment so I could do the deletion with minimal impact.