The exact same happened with me and DHL and on another occasion they issued me with the bill 6months after I received the parcel. I had to pay 40% ontop of the total of my goods which ended up £100 or so in fees.
After various phonecall disputes they deducted the admin fee (which was £1.20) and removed my "exempt from duty" items as there was some childrens clothing within the parcel.
That's pretty much the last time I'll ever use DHL if I can avoid them.