I've now tested from all the locations I previously mentioned (ie: Webmail, Outlook at home, Thunderbird Portable...)
So far, fingers crossed, everything is being delivered straight away - and has been all day.
I just need confirmation from a third-party Yahoo account that all is good for them, but at the moment it looks like it's solved - or at least fixed for now!
Thanks very much for all the help here - especially Mark and Eukhost for not giving up.
