Looking more closely, I realised the outside had faded, while the inside had stayed a darker indigo - it's most easily seen in this pic where I've removed a pocket. So instead of a fairly easy chopping up and sew together again, I ended up almost starting from scratch.
and after a test try-on, I decided to add some darts both back and front, although it's still baggy enough to pull on without needing a zip. I haven't quite sorted the waistband yet - I need some more material from somewhere - but I'm going to use the old tie from the trousers to gather and fasten it.
So far I think it looks ok and I'm extremely happy to have found a use for the material.