If you use the key in the door lock to get into the cabin then pop the fuse box cover & you will see a red plastic cap covering a terminal that you need to connect a 12 volt supply too (you can earth directly onto the door catch on the B pillar as per pic below & power wise a jump start pack or another 12 volt battery thats charged). This should give you power to press the boot switch & open it giving you access to the battery. If this fails theres an emergency release cable hidden under the front off side wheel arch liner.
If its possible might be worth leaving one of the conditioner type battery chargers connected next time your away?
