Copyright Aid

I believe Edward Hopper’s Nighthawks is public domain. He was USA based and Wiki states the artwork is free to use, but being ok in America where he was based, how would that affect legality overseas, & in the UK?

The key to answering your question, due to the US copyright law which applied at the time, is knowing whether the painting was first registered with the US Copyright Office. Doing this would have given it 28 years' worth of protection. And if the registration was renewed in the 27th year, then because of a later change in the law, the painting would have gained 95 years' worth of protection from the date it was created, taking its copyright to the end of 2037.
However the Register of Copyrights for 1942 indicates that the work was not initially registered in that year and since Hopper sold the painting in May 1942, he probably had no reason to care about the copyright. This seems to confirm that it is now in the public domain.

I read that any copyright (if applicable) was not renewed, but the 50 year/70 year thing - does this make it public domain only in the USA?

Sorry, I forgot to answer that part of your original question. Yes, the doctrine of the shorter term, as expressed in section 12(6) of the Copyright Designs and Patents Act 1988, applies here in the UK.

Thank you.