I found some nice Ponti at Tessuti so snapped it up and set up making a second striped Mable Skirt. This time I added 2cm to the centre front and back of the XL (my last iteration had 1cm added) and it fits well. The fabric does not have a lot of stretch to it so 2cm is definitely the right amount.
The waist elastic size is the same -- in fact I unpicked the 'not-right' Mabel Skirt and re used the elastic.
Pattern: "Mabel Skirt" by Colette PatternsSize: XL, sewn with 3/8" seam allowance
Modifications: 5cm elastic waistband (added 10cm to the pattern to accommodate for this), 16.5cm added to length, pegged in the bottom by 0.5cm at each seam (so 2cm in total). 2cm added to the CF and CB to accommodate for less stretch (and winter weight)
Fabric: White on Black Ponti (23% viscose, 5% spandex, 72% poly) from Tessuti