We have finally got our coolroom going just in the nick of time. We have insulated out a storage room, and installed a split unit air conditioner (thanks to Paul 'Woody').
We have then rigged up a device from the USA called a 'coolbot' (www.storeitcold.com), which makes the air con unit into a chiller to give you some extra cooling power. It's a very small gadget that only uses a tiny amount of electricity itself. For example we can easily get 4 degrees now (down from 18), although we are currently using it around 9-10 degrees.
Sounds too good to be true doesn't it? All the details are on the coolbot website, with lots of practical advice about insulation which is very important. Maybe it can't work miracles in every environment, but it certainly is for us. Many thanks to Ron Khosla for some timely advice during installation and customer support.