Continuity mistake: When the guy is in the mountains he burns some of the triffids at the fence and you can clearly see them frying. But when the camera pans across the garden later, there are still triffids, but they are all fire-free and there are no bodies.
Factual error: The geography of London is all wrong. When Howard Keel leaves the Eye Hospital (in Kings Cross) he turns a corner and ends up in Lincoln's Inn Fields (where he sees a dog being killed by a Triffid), he then exits the park and ends up in Victoria Train Station. All these locations are several miles apart.