I don't know anything about Andreas or Kidney Cleansing.. You might want to make a new topic in the kidney cleanse forum.
But as for this part:
"The last time I tried to drink for a number of days I used to get headaches at the end of the day (and I did supplement by drinking lots of water)."
Headaches are almost always from toxins not being released. Are you doing anything for your colon? Having any bms? Are you just drinking/fasting and not eating? If so, that's your problem. You should be doing Oxypowder or at least a SWF (Salt-Water-Flush) in the morning.