It occurs to me that some of the non-British members of the Forum may not be aware of what "cracking" means in the context of this thread (although by now they should have figured it out). So, in the interest of vocabulary building, I offer the following definitions.
Cracking - If something is cracking, it means it is the best. Usually said without pronouncing the last "G". If a girl is cracking it means she is stunning. (from "The Best of British - The American's guide to speaking British," at
http://www.effingpot.com/slang.shtml)
crack one off - Verb. To masturbate, with reference to the male. E.g."As soon as he has a spare 2 minutes, he's off to the toilet and cracking one off." (from "Dictionary of English slang and colloquialisms of the UK," at
http://www.peevish.co.uk/slang/index.htm)
So, happy cracking!