Hi  Chrise and Rossim. Modus has finished and my stollen is on its third rise, so I'm back at the crossword.  Thanks for your hints.  Unfortunately, the penny hasn't dropped yet.  I can't think of a large black cat unless it is one belonging to a witch.  
I've already tried the fraction approach, but nothing is obvious to me.  Is it a baking ingredient I've just had my hands in?
The penny has just dropped with theme!!!!!