Are you *sure* of your spelling? A search for "burdles gravy" on Google only brings back your question on the Answerbank! Maybe that's why it's taking so long to answer your question?
For no reason that I can ascertain, there appears to be great interest in gravy salts on the Message Board of a website for the village of Wymondham, Leicestershire. Whatever the reason, if you try this link you may find someone who can answer your question!... http://www.network54.com/Hide/Forum/47126