I've got this working now with the help of posts in this thread. Key things to look out for are:
Make sure you've got evdev.ko and that it's being loaded, use lsmod to check.
Check that everything is ok using acpid -d
Make sure you have the following files:
/usr/syno/etc/rc.d/S30acpid.sh
/usr/syno/etc/rc.d/S10module_evdev.sh
the S10module_evdev.sh needs modifying if you downloaded it
- edit MODULES_DIR="/lib/modules/"