Upon many thoughts I have decided to disassemble the extra pair of trousers. Fabric, seams and lining are in perfect condition, so I felt some ambiguity about doing so. But the purpose is to make up drawings/a pattern for a new pair of flannel trousers based on the various parts of cloth; see also Project: Flannel trousers
After this I will use the spare fabric to sew a waistcoat for the suit by hand.
I enjoy immensely the wearing of waist coats for my suits during winter – and a waist coat in the same fabric is preferable.