Q: I’m writing a report which refers to the government of former prime minister of the UK Neville Chamberlain. Should I write ‘the then-Chamberlain government’ or ‘the then Chamberlain government’?
A: I have it on the good authority of the Macquarie (AU), Merriam-Webster (US) and Macmillan (UK) dictionaries that there is no hyphen when using ‘then’ in this context, so ‘the then Chamberlain government’ would be correct.
However, as the Chamberlain government was replaced by a new Churchill government, you could probably just write ‘the Chamberlain government’ and that would be perfectly clear. It’s a different matter if you were referring to ‘the then prime minister Chamberlain’, in which case ‘then’ is necessary, though you might be able to skip ‘the’, simplifying things a bit. (As a side note, style guides often advise using lowercase initials for things like ‘prime minister’ and ‘government’ unless you’re writing about the current one.)