In reply to Gordon Stainforth:
I've only seen it once, also when I was about 15 (not in the 60s but still a long time ago). Only saw it on television, probably at my grandmother's house on an idle Saturday evening.
I keep coming close to watching it again but I always hold back because of fear of exactly what you outline above.
To be honest my interest in the film is based not so much on its "classic" status but on the fact that Cy Endfield directed it. Cy, as you probably know, made some interesting films often with Stanley Baker (my second point of interest, given that he was pretty much the biggest British movie star between around 1957 and 1964).
Hell Drivers is a fascinating portrait of the post-war working class itinerant bachelor, including one of the most terrifying performances I've ever seen (Patrick McGoohan - he's up there with Ben Kingsley in Sexy Beast and Robert Duvall in The Apostle)
Mysterious Island (no Stanley Baker) is a delirious meeting of Jules Verne and Ray Harryhausen
Sands of the Kalahari is a massively underrated "stranded in a remote location" drama with some fascinating characterisations and an unforgettable finale. Budget constraints meant that the cinematography was a bit of a let-down.
I fear that Zulu might pale in comparison to all three.