Regarding the mechanism of a dew pond, I know from personal experience that the tops of the South Downs can get pretty hot, but they can also get very chilly at night. Perhaps dew-ponds work better up high, as the temperature differential between daytime and night-time is greater than down on the plain. Dunno, really, but I see no reason why they shouldn't work anywhere.

In 2011, my guess would be that you could find more effective insulation material than straw to underlay your clay. I believe that most of the dew-ponds that failed, did so because the straw got wet or decayed, hence lost the insulation. Some kind of really thick glass-fibre or plastic closed-cell foam would probably do the trick.

I have abolutely no intention of trying to build one, however!