Back to where we started…yes it’s fine for the question that was asked 
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Many thanks chaps-feel more confident now…!
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I leave the battery connected and as already stated connect the earth to the chassis rather that the battery earth.
On my NC. there is a simple earth strap from the battery neg. post to the chassis. There is no complicated arrangement like that shown in the very helpful photograph and explanation posted by Richard FX. I have had a battery conditioner (not a Ctek make) connected directly to the battery terminals of my (occasional use) NC and 3 previous MX5’s for the last 20 years without any apparent problems.
However, I recently bought a CTek charger/conditioner for my reasonably modern Volvo which has a battery negative post arrangement similar to that shown in RichardFX’s photo. I noticed that the CTek instruction manual says to connect the neg. lead to the vehicle chassis (but doesn’t say why not to connect to the neg. battery post). Thanks to this thread I now understand the reason for the different connection options much better.
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Thanks for the confirmation about the Volvo.
Yes. NC is simple, nothing complicated, no worries, and here is a (cover off) pic of how I did it in 2015, but taken in 2017 when I changed the 9 year old Panasonic battery for a Yuasa YBX5053.
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That’s exactly how I’ve always connected both of my NCs never had any problems at all.
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Very neatly done 
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My thanks to all contributors. I am now clear in my mind about the right and wrong ways of connecting up battery conditioners in different situations for safe long term operation. A very useful thread I think, especially as owing to Covid restrictions I am now using my “daily driver” about once a fortnight.
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