With thanks to Leaflet.js forum members Matt and Simon.
I was encouraged to try again to obtain up-to-date LA boundary data from the OS. This time I obtained a set of shape files. The relevant shape file was district_borough_unitary_region.shx. I opened that in QGIS, removed the non-Scottish areas and saved the result as Scotland.qgs. I then selected and saved each LA in turn as a geoJSON vector file (with CRS = WGS84/EPSG4326 as before). So this got me up-to-date accurate LA boundaries. But these are quite large files: averaging 2MB each. (Of course Highland was much bigger than Clackmannashire, for example.)
To obtain simplified files, I selected each LA in turn, then did Vector > Geoprocessing tools > Simplify (options: ‘use only selected features’, tolerance = 50, ‘save to new file’, ‘add to canvas’) so that I had a patchwork of separate LAs:
I then selected each simplified LA in turn and saved them as geoJSON files. The resulting files are about a twentieth of the size of the unsimplified files. Yet zooming right in shows an acceptable (to me) fit to the unsimplified boundaries and coastlines. Yeehah.
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