Thanks darla, drxx, I hadn’t spotted how to use the correction letter, and now see how 15d works. Except that there are, er, about 30 *sextillion* 15ds in one of those other words… oops?!
For crying out loud! Where are the para-mathematicians - don't they know there's a mathemergency going down here?
I knew my facetious remark at the top of the thread would come back to haunt me (Oh! The humanity...).
Chambers gives the definition of quintillion as "a thousand raised to the sixth power ( 10 to the power 18)"
and I can see how that could be (wrongly) interpreted to produce the clue.
Seems to be a factor of 1,000 out to me, but I could be wrong. As far as a can see there are 31 × 10^12 km in a 6d, and 10^9 15d in a km, so should be around 31 × 10^21 15d in a 6d. Whereas the clue goes for 30 × 10 ^ 18. Could be wrong though.