The phrase 'marmalade dropper' was mentioned on Monday's Radio 4 Today programme (listen again to the interview with Guardian crime reporter Duncan Campbell and Scotland Yard detective Mark William-Thomas here. It will be available for a few more days; scroll down to 08.22).
A marmalade dropper is, according to the final paragraph of this article, "a (crime) story so gobsmacking that a reader's morning toast would drop through open-mouthed incredulity". The whole point of the Today report was there aren't so many marmalade droppers these days as most newspapers don't have court reporters hearing all the gory details of heinous crimes; instead reports are written from information gleaned from the internet.
Marmalade is a fruit preserve, made traditionally from Seville oranges. It is typically spread on toast at breakfast-time. It is an old word and comes from the Portuguese marmelo, meaning quince. Marmalade was originally a jelly made from quinces.
Jonathon Green's Dictionary of Slang contains the entry 'marmalade country' which supposedly refers to Scotland, but I have never heard that before. Marmalade is associated with Scotland, though.
I know I constantly bang on about the number of words in the Oxford English Dictionary which mean 'prostitute' (such as mystery), but there's one on the 'marmalade' page too. 'Marmalade-madam' was used in the late 17th and early 18th century in this sense. Another nice word on the page is 'marmalade-eater', meaning a daintily brought-up person (now obsolete).