The solution (for me at least) was to increase the memory limit in the php.ini file.
For anyone else having the same problem look for the line "memory_limit = " I had to set this to memory_limit = 128M (I tried 32M and 64M and neither worked - it may be possible to do it somewhere between 64M and 128M).
You'll need to restart apache after making the change.
You beat me to it, was just going to point out the min server requirements and see if you are meeting them but you had to increase further.
Extract from system requirements.
Set the maximum memory to at least 48 mb. SilverStripe can be resource hungry for some intensive operations. Note: You can sometimes increase the maximum memory by adding a â€œini_set(â€memory_limitâ€,â€48Mâ€);â€ line to sapphire/main.php