it could be an inner tie-rod or track rod end - jack the front of your car up and rock the front wheels to find any free play; you will usually feel a "clicking" as opposed to nothing (sorry if i have explained this incorrectly!)
if you don't feel confident enough to do this yourself, take it to another mechanic to get his opinion - if the garage who tracked your car missed something as obvious as that, i'd have my hand out for a substantial refund!
i have to replace an inner tie-rod in my baby tomorrow - looks easy enough, everything is accessible (symptoms same as yours, and it knocks when i hit a bump at higher speeds)
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1998 318is, 1.9litre m44