18 April : I got some Blackboard Paint from Screwfix, and have painted the back of the Sharp PV panel, so that it can radiate inwards towards the steel radiator. The sun was out, and as soon as the paint was dry, it had a surface temperature of about 60ºC when the sun was shining (even though air temperature is still cool). This bodes well.
The steel radiator was also painted, and those pesky fins are a real pain to paint. The front surface of the radiator has to pick up heat from the back of the PV panel. The fins at the back are important, as they are mainly to pick up air temperature, but their black surface will also project surplus heat into the brickwork which will act as a short term storage medium.
The polycarbonate was ordered on line from LIV Supplies, and so there's no going back! I have enough aluminium and polycarbonate to make a 'faux-PV Panel' that is dimensionally identical to the real one, and can be interchanged - so I can test it out with a clear over panel allowing the radiator to see the sun directly. And I can try with a piece of black painted polycarbonate to see if that will project heat into the sunbox.
The sunbox mark 2 will be called Savita, which is the female equivalent to Surya - but like a woman, vastly more mysterious and complex than the male version.