Charity shops crammed with unwanted Shades of Grey

Sarah Coles
50 Shades of Grey
50 Shades of Grey

Charity shops are apparently receiving thousands of copies of 50 Shades of Grey as donations - but nobody wants to buy them. In many cases when the shops finally rid themselves of a copy, they'll get two more to take its place.

So what is driving this odd phenomenon, and is this the least welcome charity donation?