I'll see what I can come up with but it's a bit like the blind leading the blind! I'm no electronics expert, I usually just find a way to do what I need to somehow

, Maplin do various 12v delay relay kits btw.
Like you say instantly pulling the power to everthing is the best way if you have a TEC. There was a good article about an instant PSU shutoff system in this forum about a year ago that used a temp sensor so it would shut down if the heatsink fell off the CPU. That circuit could be made to work with the flow switch I guess.
How have you done yours btw, with the same flow switch as me?