In my experience, some stuff gets through, some doesn't. I think it also depends on what the supplier puts on the box - gifts for example, are not subject to tax. You've also got to think about the "holding charge" that nice companies like Parcelfarce add to the duty while they wait for your payment, often more than the bloody import duty. There, glad I got that off my chest.
I've had good and bad experiences, but absolutely no idea of the rules. Maybe I should have said that at the start. Good luck and keep us posted...
EDIT: Listen to the Hamster!