I restored my backup, going back to v1.6.1.0 - everything OK.
I then upped the memory to 512m in admin and root directories - cant hurt, can it! everything still OK.
I then applied the suggested update to the config.inc.php file and immediately started getting the WSOD - so took it back out.
Everything has been working fine and having updated the products that needed it I decided to retry the upgrade.
1.6.1.2 had come out by then, so updated to this version.
Everything this time seems OK!!
Could be a difference between 1.6.1.1 and 1.6.1.2 but I suspect that the suggested update to config.inc.php was the problem.