... is, sadly, a rectangle.
Due to a combination of factors mostly involving not paying attention, my rectangular tension square in garter stitch was 8cm x 12cm rather than the (still too large 12cm x 12cm) that I might have ended up with if I'd used the stocking stitch suggested.
Yarn: Phildar Alibi Ob (61% Acrylic, 26% Wool, 13% Viscose)
Stitch Used: Garter
Number of Stitches: 25 (+0)
Number of Rows: 34 (+0)
Washed? No (yarn instructions - machine wash, 40oC)
Intended Pattern: An adaptation of 'Silk Evening Scarf', p66 of I Love Knitting (Rachel Henderson)
Better luck next time?