[I was wrong - see _az’s reply below]
Thanks _az.
It seemed such an obvious question that there had to be an ‘official’ answer.
I tested the “mozilla” “modern” config with the ssllabs online test : https://www.ssllabs.com/ssltest/index.html
=> the mozilla config leaves out the chain file
=> the ssllabs test wants it (without it, got a B, with it, an A+)
Might be worth recommending the “mozilla config” it in the Letsencrypt documentation.