I have received this welcome communication from Weston Library: I've tried it and it works!
"Ancestry are allowing us to use ancestrylibrary.com from home temporarily while the libraries are shut.
As the access needs to be secure we are fortunate that LibrariesWest libraries are all subscribers.
If people log in to www.librarieswest.org.uk with their library card and PIN. Look for the words in the middle of the page
Access to Ancestry Library Edition and click on the heading. This should take them to Ancestry. If they just go to the home page without logging in it won’t be visible. People need to use this link each time they want to use Ancestry as it is expects people to arrive at the website from www.librarieswest.org.uk via the link.
(P.S. It will say Bath Central Library rather than North Somerset Libraries)."