So I did a web search and looked in my pantry and worked with what I had. I mixed together:
1 tbsp citric acid (I originally started keeping this in my pantry when I did the FAILSAFE diet, but as someone without a citrus tree in my backyard, it sure comes in handy)
1 tbsp iodised cooking salt
1 tbsp bicarbonate of soda
I used about half a tablespoon of that mix in the dishwasher. I've used it for three nights and the dishes are CLEAN (and not salty). I use vinegar in the rinse aid compartment, always have done.
I don't think I'd mix up a big quantity of this, as it's already starting to clump in the container. Nor do I think I'd use it every night - most reviews of homemade dishwasher powder I've seen claim it works for a while and then a cloudy residue forms on the dishes. But I might start alternating it with the expensive eco-friendlier tablets I normally use, and it's DEFINITELY good stuff to have on hand in a pinch. Otherwise I'd have had to hand-wash the dishes until I got to the shops. Shock, horror! (Seriously, I hate doing dishes.)
Photo licenced under Creative Commons, taken by jbcurio