Hi and thank you, @AITpro Admin keymaster.
In case anyone else has this problem, the steps I took were to go to the BPS pro tools page (in website dashboard) and click delete for both online and offline base64 decoders. After further advice from BPS support (thanks) I have also deleted the specific zipped files mentioned in my post above, via ftp. I am hopeful that I will no longer get malware alerts from my backup plugin!
I am extremely grateful for the excellent support from AIT pro admin. Cheers Jenny