"So you reckon it doesn;t always work ?"

Do you mean OpenScad Customizer?
If so:
That works consistently for me, but has the limitation I described in my opening post.

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Or do you mean thingiverse Customizer?

If so:
1. I only tried it 3-4 times but had erratic behaviour (which could well have been my error as a rookie!). Haven't tried since. Would be interested in feedback from you or others if you try it. And perhaps it would be a way of avoiding the file saving issue, by returning to thingiverse to make a succession of edits.

2. As I said, I read quite a few posts that it had periods of being inacessibel/unusable. For example, google
'thingiverse customizer not working 2020'