In reply to ianstevens: The FRA website rules state:
Best Practice Kit’ is the mandatory minimum for AL, AM and BL races, and Race
Organisers may require it to be carried at other categories of event.
‘Best Practice Kit’ comprises:-
 Waterproof whole body cover, hat, gloves, map of the route, compass, whistle, emergency food.
Your waterproof whole body cover must have taped seams and an attached hood.
I guess that precludes a bivvy bag.