It was a lot easier once I got back to them. They are not the greatest sewing-wise (a little wonky), but they're oh so comfy. Elasticated waists, how I love thee!
The main things I took from the advice were: lots of pins, baste well and go slowly! I'm also busy looking into starch spray, and sewing with paper, for when I work with my lovely new slippery and silky rayon fabric. But for now, here's the details of my practice refashion...
This was a very simple re-size. I brought the sides and inside led seams in by an inch. I managed to do the sides by using french seams. The inside leg I just sewed straight, then used a zig zag to secure the edges.
I've tapered them in dramatically compared with the 80's original. I am looking forward to wearing them as they are so light and airy. Perfect for this heatwave we are experiencing.
I think they are pretty shapely, considering, but my husband thinks I look silly in them! Arrggh! I'm going to wear them anyway, of course.
Thank you again for all your advice, The online sewing community is fantastic, and so supportive. I love this bloggin' lark.