I added a separate casing for the back, 5 belt loops. The front has been shortened by 5cm (2 inches) and the band reattached. The back was shortened to match the chalk markings and the new casing attached.
Front with flat waistband
Here's a close up of the fabric, the pinstripes are made of double stripes alternately in white and pale grey, with the pale grey also having an outer stripe of an invisible thread which sometimes catches the light.
The fabrics was from deep stash, a 2m cut which seems to be a nice quality wool mix suiting. I have no recall whatsoever where I bought it.