I’m planning on charging a spare battery in my boot easiest way at moment is to plug into 13 pin socket run cable in boot and attach to battery. Do I use pins 10&11 or pins 9&13 to charge the battery
Use 10 & 11. You do not want the leisure battery connected when you start the car as it will draw a huge current when it does. The battery and cables just won't take it.
How long will you be away? If you trust the spare battery, can't you just charge it at home and simply take it with you in a disconnected state?
Bertie.