worked to the bone

this charge is a bolt out of the blue. i earn over the threshold my partner does not. she has a child that lives with us and claims child benefit. The father of her child pays no maintenance as he claims to be unemployed but works as a builder !!.and does very nicely. i pay £700 pm for my 2 children from a previous relationship. Our finances are separate. i don't understand why i should pay back someone else child benefit. And the letter for the charge turns up 6 months into the next tax year and tells you it can be backdated for years. Doesnt seem a very fair system