dundee,
to shock off is to chill rapidly to stop the cooking process. you can do it with an ice bath which is just ice cubes in a little water. in the case of shellfish, since you don't want to lose any of the liquid or add water to them, a big bowl of ice water with a smaller bowl set into that will work or you can even put them into your freezer but watch them carefully since at this point you don't want to freeze them.
by the way we get green lipped mussels here but they come frozen. never seen a tough one yet. i think the freezing somewhat breaks down the fibers.
jack
ps. you sir are braver than i!!!!!! poms!!!!! good lord let's call them roast beefs like the french lol
pps. hey by the way thanks for this thread. it made me go back through my old sous chef's notes i made years ago working in a convention center. sure would hate to forget how to cook continental