Well, I don't know what's going on...
I thought I'd made a typo in the php.ini file when I saw it said 128M in the memory allocation so I changed it to 128MB.
I then re-downloaded php onto my Mac and checked the php.ini.recommended file. This said 128M.
When I changed the file back to 128M on my Ubuntu box and restarted apache2, the ISPConfig login page appeared!
Nothing really changed from previously as it said 128M before...
Go figure :-)