Terry, I reach out & pick up Concise Oxford, nearest dictionary to hand. Here is the entry exactly, omitting only stress marks which too much hassle. Note order of entry:

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marquis, marquess, n. nobleman ranking between duke and (in the UK) earl or (elsewhere) count.
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Know better, do you? Then I hope it keeps fine for you. Try any dictionary and you'll find the same. I admit most directories give the -ess form, the other being more old-fashioned. But both are acceptable, & if you think not, then I don't care either.

Anyhow, the Longleat geezer is one of the nation's most notable loonies! I shouldn't rely too much on him as an exemplar if I were you...

Regards

~M~