It seems apple aren't keen on Spire and it's growing popularity. So much so that they're enforcing more rules to break it for Spire users.
Previously, you only needed Spire, a host and a certificate and you were away. Now Apple have added a "SetActivationToken" which has disabled Siri for almost everyone on illegitimate devices.
A temporary (hopefully) workaround of deleting "com.apple.assistant.plist" from "/var/mobile/library/preferences" on the device appears to resolve the issue for now, but will this become another game of cat and mouse for apple, just like in Jailbreaking?
No doubt we'll soon find out!