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"... it's only very rarely that I have seen a case fail because it has spilt at the neck. I have seen signs of pending case head separation mostly in .303 Brit."
You're setting your case shoulders much too far back, that's what causes excessive stretching and head seperations. Well, that and a 'springy' action, especially one with rear locking lugs.
"All the loads I did for my 303 Brit and 7.62x54r I neck sized only and had no issues."
Neck sizing doesn't set shoulders back at all; that one thing prevents head seperations but it's not that necking, per se, is doing anything special.
"Maybe I haven't reused my brass enough to start seeing failures."
You're seeing a failure it seems, but a worse one. Correct your FL sizing and you'll see neck splits, they are harmless but head seperations can be serious.
I've not seen that "all the major reloading companies" say necking improves accuracy. Some loading manuals have articles written by paid "experts" that say so but, in 48 years of reloading, I've NEVER seen any loading company make that claim; in fact some loading companies specifically say otherwise. If you still think all gun writers are the last word for anything you're in for more disappointments.
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