Thanks Doug – I don’t have access to the Kew book unfortunately.
Below is a close up of under the spore sac.
The wikipedia entry for G. pectinatum (admittedly possibly not the most reliable source of information) says
“The tough and membranous endoperidium comprising the spore sac, purple-brown in color and 0.5 to 1.5 cm (0.2 to 0.6 in) tall by 1 to 2.5 cm (0.4 to 1.0 in) wide, is supported by a small stalk—a pedicel—that is 3–4 mm long by 7–10 mm wide and which has a grooved (sulcate) apophysis, or swelling. This ring-shaped swelling is made of remnants from a tissue called the pseudoparenchymatous layer.[16] When fresh, the pseudoparenchymatous layer is whitish in color, thick and fleshy; it dries to become brown to dark brown while shrinking and often splitting and peeling.[13] “