The problem seems to be PuTTy itself.
I re-enabled Telnet on the server and used the Windows Telnet client to access the server.
Also downloaded an evaluation copy of VanDyke SecureCRT and that worked for SSH.
Sorry to bother y'all. I knew it was a stupid question.
I've now found out what I was doing wrong with PuTTY.
I had downloaded the zip file with PuTTY and all its related programs, and had opened the file with WinZip. I had run the PuTTY executable from within WInZip. It worked fine when I first unzipped the file to a directory, and ran Putty.exe from the directory.
Well you never know, there might be somebody else as daft as me that this might help.