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There seem to be two acceptable answers for 29a (both of whch satisfy the definitiion and the wordplay) with no indication which is the preferred one. Or have I missed something?

I think in singular form there is only one possibility for the definition, which rules out the alternative one that fits the wordplay.

Chambers gives both spellings as the singular form of the definition and both spellings for the wordplay.

Re 29a. My printed copy of Chambers gives both spellings of the definition as old/rare variations of the shorter word.
My Windows and Android apps only support one spelling.

Chambers Word WIzard gives both spellings. According to Chambers 12th Edition, the first spelling is 'old', while the second is 'rare'. Since the clue says 'unusual', I will go for the second option.

Seems there may be differences between Chambers versions.

Chambers 2016 (Revised 13th edition) only has one spelling which is prefixed with “rarely”.

This is consistent with the Chambers app.