Had a knock in the front left of the car I just bought. After visibly checking LCA bushes (already replaced with Pedders items in the not-too-distant past), started going through a list of other possibilities that I got from this thread; strut nut, crossmember bolts, and check spring location. (Very helpful thread btw.)
Luckily enough it was the next thing that I tried - the top nut hidden under the cap was loose. I held the bolt from the strut in place with a 6mm robert allenby and tightened it back up. You'll need a 17mm spanner with some level of offset on it but other than that it was an Easy Fix.