Well, this is embarrassing!
Approximately 3 minutes after I posted, I did some simple "back-to-basics" network diagnostics and found that the Pi had lost Internet connectivity...
Just before I went out last night, I set up the Pi with a static IP rather than a DHCP & rebooted. When I got back I had forgotten that fact and when I discovered the typo I'd made on the gateway IP, I could have kicked myself!
Typo on gateway corrected in "/etc/network/interfaces", then "/etc/init.d/networking reload" and all my data upload issues are gone - what a fool I am.
Should be a nice easy git of googling for any other numpties who have similar issues in the future :)
Cheers,
--
ChrisP