I think I've tracked down the Guardian news story you're talking about Ed. It looks like some 'scientivists' associated with the "Skeptical Science" (SkS) blog have produced a 'widget' that can be inserted into webpages:
This 4 Hiroshima bombs per second idea came up a few months ago when the ringleader of "Skeptical Science", John Cook, announced it at some climate change meeting (apparently the inventor of the idea was actually James Hansen in 2012). WUWT wrote a blog post at the time which debunked the claim:
WUWT estimate that the 4 Hiroshima bombs per second energy figure is equivalent to 0.6 Watts per square metre when you take account of the Earth's surface area, and give the sums for it. So basically the SkS gang are taking a very small amount of energy per unit area and relating it to a much larger amount of energy concentrated over a much smaller area.
WUWT also quote a paper which estimates the uncertainties in the various heat flows in the earth - sun - outer space heat radiation flow diagram. The net amount of energy coming in at the Earth's surface is given as 0.6 W/m², but the uncertainty on that figure is estimated as ±17 W/m². [It isn't stated whether this uncertainty is a standard deviation, or a range, the latter often being taken as 2 to 3 times the standard deviation, and the detail can't be checked because the paper is sitting behind a paywall].