Dito what Rich says, I usually use 2 pieces at a time, leaving one in a tub of (as warm as possible) water. Then I just switch every panel or when the current clay starts to go cold.
the regular one is like a quid cheaper too. think i'll get some for a tenner.
is it advisable to do it in the cold tho? i know not in direct sunlight (that wont be difficult in the UK lol)