[GordonS]

> subject to the liberal OSM licence

Not to piss on the OP's parade, but, while liberal, the OSM license does require you to to give proper attribution to OSM in such a way than users are likely to notice it - I don't see that here.

Would be nice to get the OP's comments on this?

[mtmail]

I see "(c) Mapbox (c) OpenStreetMap" in the bottom right of the map preview, so I assume it will also be part of the export/print. "OpenStreetMap contributors" and another clickable link in the footer would be better but I see reasonable effort in/on the end product.

edit: reading through other comments it's possible the author added the attribution only recently (after OPs comment)