So I configured Terminal.app to use Option (Alt) as Meta key: Terminal => Preferences => Profiles => Keyboard => Use Option as Meta key

But wasn’t working for the key-bindings in my .vimrc:

nmap <M-j> mz:m+<cr>`z
nmap <M-k> mz:m-2<cr>`z

So I found this article: Fix meta-keys that break out of Insert mode and ended up with the following snippet for my .vimrc which fixed my Meta-key problem:

let c='a'
while c <= 'z'
  exec "set <M-".tolower(c).">=\e".c
  exec "imap \e".c." <M-".tolower(c).">"
  let c = nr2char(1+char2nr(c))
endw

Everything is easy when you know how!