well thats tosh.
whats happened is the rear deralier (I call it a mech for short) has been incorrectly adjusted or been hit with something (maybe bike fell on that side would probably do it with a cheap bike).
whats mean't to happen is the metal tab between the frame and the mech is mean't to break FIRST.
but on cheap bikes they often don't have this part. its called a 'mech hanger' thats what th manufacturer has stated, that it was mean't to break first, but sometimes they don't.
if going anymore than about 8mph, and you weigh more than 8stone, you will cause some mega damage when it goes in the wheel, it will either explode the wheel, damage the wheel beyond resonable repair, or damage the frame.
in this case, the frame has taken the brunt (it'll be them cheap, heavy, but tough steel plain gauge spokes it was fitted with in taiwan).
the only influence youcould have put on it to cause damage would be hitting the mech with a large stone/boulder/tree stump whilst out riding on it.
leaning it on its side with the mech on the ground
transporting it back home damaging it some other way.
JJB should go to hell if they won't even look at it properly