I found these instuctions;
http://nicolebluto.com/node/20
I installed php-apc;
Code:
aptitude install php-apc
Then I added these lines to php.ini;
Code:
extension=apc.so
apc.enabled=1
apc.shm_size="64
apache 2 restart;
Code:
/etc/init.d/apache2 restart
However, after this was done, I was left with a blank page at my web address.
I had to remove php-apc and restart apache2 to restore my web site.
This was done very early morning

so I probably have missed out a very simple step.
Any advice would be much appreciated.