Mostly when talking about biscuits. It's when something is very easy to keep eating, but isn't actually addictive.
That's a line form Peep show.
Honestly, if I hadn't worked with so many Brits while living abroad, I don't think I would know it. I have never heard mardy, though I have heard the word "manky" (is that the right spelling?). There seem to be quite a few interesting words in colloquial use in England (not to mention the rest of the UK) that I think Americans would enjoy using.
I've only ever heard manky said, but that looks right to me! English has some lovely words. We use SO many american-isms though, couch and trash and basically adopted into our language now. (Sofa and rubbish are the proper words, of course) I always get quite annoyed that Americans don't have kettles. WHAT IF YOU WANT TEA.
Heh, some of us do, but generally not the electric kettles. Some people microwave their water for tea (I know, I know). It is curious that American media infiltrates so many English speaking countries, but very little English language media from outside of the US comes in. I used to play that up with a friend of mine whenever he would complain about America, by talking about how much I enjoyed Benny Hill.