Has the pronunciation of ‘heretic’ changed over time?

Note that accented syllables can vary greatly especially between US and UK versions.

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I'm British and I have only ever heard ‘HERetic’ for the noun.

If I heard ‘heRETic’ I would assume it was intended as an adjective not a noun - an abbreviation of ‘heRETical’

Examples

Pelagius was a British HERetic. (noun)

Pelagius was a heRETical Briton. (adjective)

Pelagius was a heRETic Briton. (not standard usage but I would assume it was a form of "heretical")

I was wondering if ‘heRETic’ is or has ever been correct, or v.v. ‘HERetical'

No.

Lawrence is right in thinking that the stress in English is typically placed on the antepenultimate syllable.

Although heretic ends with -ic, it's a noun. I think the -ic in 'heretic' is not a suffix.

Words that end with the suffix -ic are usually stressed on the penultimate syllable (second last syllable).
Examples: Historic, ecstatic, empathetic etc.

Most nouns are usually stressed on the first syllable. So 'heretic' is stressed on the first syllable and is pronounced /'herətɪk/.
On the other hand, heretical is an adjective and adjectives that end with -ical are usually stressed on the antepenultimate syllable (third-last syllable), so 'heretical' is stressed on the antepenult: /hə'retɪk(ə)l/.

Other words that end with -ical and are stressed on the antepenult are historical, empirical, cynical etc.