The reason for the fact that the assemblies had different versions is that I had already download the FTP pack about 2-3 weeks ago and used it OK but then my customer asked me to add SFTP support so I installed SFTP 2015.
I noticed today that the FTP pack is now 2015 R1 as well so I have re-installed both together and all is well.
Thanks for the tip about 'Universal FTP/SFTP client overview' but I had already developed a late binding solution for my project but I will see what works best.
Many thanks again.