In case anyone else using the MNRAS LaTeX class has been tearing their hair out inserting line breaks manually into long author (and institution) lists, using `\newauthor`, here's how I did it:

```\author[J.~Bloggs et al.] {\parbox{\textwidth}{J.~Bloggs,\$^{1}\$\thanks{E-mail: \texttt{j.bloggs@no.where}} J.~Bloggs,\$^{2,3}\$ J.~Bloggs,\$^{3}\$ J.~Bloggs,\$^{4}\$ J.~Bloggs,\$^{5}\$ J.~Bloggs,\$^{6}\$ %... J.~Bloggs\$^{99}\$ and J.~Bloggs\$^{100}\$}\vspace{0.4cm}\\ \parbox{\textwidth}{\$^{1}\$Department of Something, Somewhere\\ \$^{2}\$Department of Something, Somewhere\\ \$^{3}\$Department of Something, Somewhere\\ \$^{4}\$Department of Something, Somewhere\\ \$^{5}\$Department of Something, Somewhere\\ \$^{6}\$Department of Something, Somewhere\\ %... \$^{99}\$Department of Something, Somewhere\\ \$^{100}\$Department of Something, Somewhere}}```

Next: a Python script to generate the above from a simple list of authors and affiliations.