Hi Matty,
Unfortunately, we do not have it in the Debian repositories. That said, compiling from source should work. I can see how you got confused by this, and for that we apologize.
These steps are assuming you want to use PHP 5.5. If you want to use 5.6, just replace '55' with '56'
decompress the tar and cd into it.
/usr/local/lsws/lsphp55/bin/phpize./configure --enable-memcache --with-php-config=/usr/local/lsws/lsphp55/bin/php-config
make
make install
Once installed, you need to add a line to your php.ini file in the Extensions section:
extension=/path/to/memcache.so
As an example, mine was installed to /usr/local/lsws/lsphp56/lib/php/20131226/memcache.so
Once added, just restart the server and check the php info page to confirm that it was added.
NOTE: The configure step may fail if you do not have the dev lsphp package. It will say something regarding missing a php_sessions.h file.
apt-get update
apt-get install lsphp55-dev
Please let me know if you run into any problems,
Kevin