Someone who lived in Belsize Square around that time - late 1960s/early 1970s - tells me that the Witches Cauldron was "almost opposite the Belsize Tavern". If that is so, then I suppose it was on the site of what became Conrad's Bistro or of the restaurant to the right of it, or both. I forget the name of the other restaurant; perhaps it was called the WC...
   The two restaurants shared the same proprietor, a
formidable Ulsterwoman called Daphne, who ran it with her toyboy Joel, a deserter from the French army. They also shared the same kitchen and the same chef, 'Elvis', but not the same prices. Conrad's Bistro had an upstairs section, and the whole of the cellar area, so the other place was quite small in comparison. These two restaurants were there when I first went to Belsize Park in 1970 and still operating in the late 1970s, so I presume the WC preceded that...