Just be honest with them. Say you had no ideas what to talk about so you went on to Pie and Bovril and asked for ideas. As expected, you got some excellent responses, such as Shakespeare, Quantum Physics, Landscape gardening in Peru, the effects of climate change on the iguana population of Papua New Guinea, geophysics, carbon dating of the ancient Caledonian Forest, America's on-going ban of haggis, malt whisky, an island in the Atlantic called Tristan de Cunha, scrabbled eggs, the ozone layer, washing powder, shinty and aardvarks. That should keep it going for five minutes.
Good Luck.