Looking at the options sheet for my car (191fsi combi) it suggests there is the option of having two batteries. The current one (no pun intended) is in the boot. I assume an extra would be under the bonnet. 
 

Has anyone seen/done this?

 

 I have a spare battery and occasional parasitic drains I am yet to trace. Am about to swap head unit to upgrade. 

Answering my own question… 

 

There is no room under the bonnet. 
 

The best option would be to install where the medical kit goes (opposite existing battery), cut boot liner - essentially mirror existing battery and wire in parallel.