vauxhall corsa 1.3 cdti 2007 steering click/feedback at low speeds?

since day 1 my corsa has produced a kind of feedback/click at low speeds (below 20mph) going forward and backwards when the wheel is turned. Difficult to describe its a kind of feedback, slightly notchy feeling as the wheel crosses a certain point. For example i can experience it, go slightly past that point then come back & it will occur again. Local dealership has tried alsorts, changed the rack, column & the other steering componenet which escapes me, all to no avail. Anybody else experience it or know what it might be? thanks