Steady on chaps!
If you really want to nail someone for low flying, you'll need something more reliable than " I think it was 1000ft, 500 - 900ft".
I've been in aviation (ATC) for 25 years now and I still find it very difficult to estimate the height of a moving aircraft.
I agree that low flying outside the confines of airshows and taking off/landing is irresponsible and invites unwarrented risk, but if it ever comes to court action you'll need solid evidence, preferably from a video camera which will give some perspective against surrounding terrain.
The idiots who fly in this manner will usually laugh in your face until you confront them with such pictures.
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It wasn't me.