I've had a few issues with my 2008 90, i work away for long periods so my Defender can be stood for 4 months at a time, on the odd occasion a family member gives it a run or needs to borrow it, the battery will either be flat or very low, this in turn creates a problem with my alarm and immobiliser.
Is it possible to fit a battery isolator, with some form of fuse to allow just the alarm to be powered permanently, but hopefully not drain the battery?

You might as well get a trickle charger and keep the vehicle running. At least the vehicle will be safe and alarmed. The battery will still drain because the alarm will still be active via an isolation source.

If you leave a connection of any sort it will still lose charge, just disconnect the earth.