I gave Proviation the chance to change their webpage to reflect that they don't have the product in stock. However, they seem to believe that their practice is acceptable, so (as I told them I would) I have now reported them to the Advertising Standards Authority, to let them decide.
If your problem is as you describe, you should also report them to the Trading Standards authority in their (not your) area.
As I understand it, the Advertising Standards Authority can only make them stop misleading advertising, the Trading Standards Authority have a very much wider remit.