Paraffin wax may be a good way of storing heat because you get the latent heat of melting (solidifying). Otherwise if your main requirement is domestic heating: water has the highest specific heat capacity of any substance.

I don't suppose you have a web reference that validates the suitability of materials for the purpose of heat storage. I am hoping to find a material that is suitable for storing a lack of heat - some kind of eutectic fluid perhaps - that will maintain a low temperature for long periods.

What temperature do you want? If you go for the eutectic mix of salt and water, you can maintain a temp. as low as -21 ^oC. Warmer than that just mix less salt; up to the perfect solution (no pun intended) of no salt for 0 ^oC.

http://en.wikipedia.org/wiki/Eutectic_point#Other_eutectic_mixtures

Was looking to maintain a temperature of +4^oC so I guess water would be the obvious solution

Yep, +4^oC is probably the easiest to maintain, because water reaches it's maximum density at that temperature, so you only need to place your sample at the lowest point of the "system" and make sure you keep some ice on the top. Just like a pond which has ice on the surface.

Not sure why people wandered off into the woods to check out cold stuff. Just a little irrelevent to the thread, i think. Anyway, a guy in the USA offered to do a nice diagram of the low tech dripper tracker and it is at http://solarcooking.wikia.com/wiki/Tracking My ugly drawing of the clock based tracker is lower down on that page too. Either tracker is probably relevent to solar panels as well as to solar cookers. From what I read, tracking improves the output of solar panels by at least 25% BUT tracking is usually too expensive to be worth it. These trackers are really cheap. My biggest failure in making the accumulator was the reflector. I didn't make the kitchen foil smooth enough. Anyone know if mylar is stretchable? Brian