It's turned out to be a temporary fix.... Received 2 more similar messages, different file names but otherwise the same structure. I decided to try to add the following to my Anti-Virus exception paths to see if it would get around it: C:/Users/Patricia/Documents/R/win-library/3.5/* - but it didn't work.
I was able to download another package without a hitch.... and Windows Defender has been turned off. Any advice appreciated, thanks.