I bought one of these years back (in the 1990s) from Fitchetts and the lower brackets were supplied, but not attached, just taped inside the valence. I was told at the time that you fit and gap the valence, then position the brackets to brace the valence whilst in the proper location against the chassis, and tackweld them onto the rear of the valence. The car was resprayed before they were ever fitted.
Regarding the bumper brackets I'd guess it's a one-size-fits-all for Heralds and Vitesse; the unit is supplied as a basic and you're expected to add the bits for your specific model.
Give Chic Doig a ring, he'll either supply them or make them if no-one else that you've tried has them.