I am another victim of the mysterious steering clunk. I have similar symptoms as others: low speed clunk, happens more as the weather gets warmer, etc, etc. After taking off the wheels, removing the struts, jiggling various control arms and links, checking the torque on various nuts, I think I have isolated the problem. The left arm of my steering rack has an ever-so-slight bit of up-down jiggle (AKA "clunk"). This is consistent with my feeling the little knocks over bumps, but having no significant decrease in steering response. Conclusion: I think my left inner rack joint is worn.
I was bracing to pay for an entirely new rack, but found that Seriously Lotus sells replacement steering rack arms/joints for a fraction of the cost. Their description says that wear on the inner joint is "common" so it may be what others have experienced. I'm going to order the arms and will post an update once installed. Wish me luck!