Ridiculously long winded way of buying ebooks from Sainburys and installing them on the Nook in the instructions on the Sainsburys site, and it didn't work for me anyway, here is a simpler way:
You still need to get an Adobe ID and download the Adobe Digital Editions software for your PC, so do that first and also authorise your PC and Nook.
Then download the ACSM files from your library on the Sainsburys site.
Then drag the ACSM files into Adobe Digital Editions and the book will appear as an epub file in the My Digital Editions folder in My Documents, then just copy it across to your Nook.
After that all you will need to do in future is download the ACSM file, open Adobe Digital Editions, drag the file into ADE then plug your Nook into your PC and copy it across from My Documents.