I finally found a way to update the firmware in my Celestron AVX mount after struggling in vain with non-functional drivers in the 9-pin Serial-to-USB adapter I bought from eBay - in the end I used an old laptop I had in the cupboard which I recalled had a serial port on it. (Hence justifying my tendency never to throw away old-tech
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This (combined with more careful choice of alignment stars) seems to have fixed my polar alignment problem.