luceos , I've moved composer.phar into the same directory and that allowed me to run the remove command without errors.
iFaisal Davis, unfortunately I'm on a shared host so I don't think I can install Composer myself.
Update
I've managed to run the remove command without errors, but I'm now unable to run the following command
composer require $require
When looking into this, StackOverflow suggests that this is because I am using PHP in CGI Binary mode not CLI.
I'm not going to pretend to know what that means, but I know it suggests I go to my host and raise a support ticket.
Thanks for your help and replies, I'll update this thread if I get a reply on the off chance someone else is struggling with a shared host ?