Hi Nick,
I'm going to float this product as I've not seen it discussed much, but I've been using it for the past year or so and am generally pretty happy with it:
http://en.rf-gsm.com/products/RF_V1...ar_GPS_Tracking_System_motor_tracker_590.html
Having previously used a separate alarm and GPS tracker, it's nice to have an integrated unit that uses a key-fob remote for arming, rather than just the usual SMS commands.
(The alarm and tracker are discrete units but the remote activates both functions simultaneously.)
The alarm/tracker is the usual type that takes a SIM card and sends an alert when detecting movement. It can then be sent commands to obtain position or be tracked live etc.
It's not perfect though; the mobile app for it is flaky (not a big deal, as you don't need to use it) and the tracker stand-by time is short (a day or two at most, if the bike is not used. This can be remedied though by connecting it to a cheap portable power bank if you know the bike will sit for a while).
HTH