Duct tape over the cat flap does seem to be working. I am pretty sure it was a cat. I heard it come in and run out quickly one night at 1:00 am.

The initial thread was to alert any cat owners in the area, who's cat might be gaining weight, that they had a midnight raider.

But I do appreciate all the 'infestation' advice. Don't fall out over a few pence worth of Sainsbury's brand cat nibbles chaps.