This was fine, like a lot of Riverford recipes it’s missing information and makes too many assumptions about what you know.
What it wants you to do is make a 7-8cm diameter cylinder out of the squash, but it doesn’t say that so if you get a particularly chunky squash neck you end up eating a pound of squash to every teaspoon of mushrooms.
Ideally I’d do this by roasting the squash till soft, pushing an 8cm diameter cutter through the neck, chilling overnight and then searing the squash as per the recipe.
Alternatively ditch the attempt to make a massive vegetarian centrepiece and make sensible individual wellingtons with a good amount of squash in them cut from the roasted piece as per the recipe.
