OK this one is solved - apparently!
After adding in the 'PHP_INI_SCAN_DIR=$VH_ROOT/ini' - the blank page was a change factor at least - so I knew that something was going on with the php.ini in that folder.
It turns out that the phpinfo file I had been using used short tags - these are enabled by default, however not in the php.ini file that I had copied into place. After changing the phpinfo file - back it came.
The interesting thing is that although the phpinfo is still telling me that no configuration is loaded - changes made to this site based php.ini DO show up and are made. I changed upload size, corrected a timezone issue, all these php.ini changes worked.
It therefore appears that with LSWS & PHP (I tested both 5.3 and 5.4) - there is no way to tell if your php.ini is actually loaded!!!!
I guess that's a bug eh....?