Thanks to Nony at a A Slob Comes Clean I now know that my dishwasher should be put on every night without fail, be it full or not. It really does make a difference even though it feels ‘wrong’ to run a half full dishwasher.
I tried not to. I tried to save water, to save dishwasher tablets (15p each if you’re interested). I tried to finish filling the machine the next day and ended up having to run two loads the next day or even worse *gasp* wash the dishes by hand!