Electrics Leisure battery wiring (cable and fuses)

Hi peeps,

I'd like to add some new wiring to my dash direct from the leisure battery.

Currently the stereo, etc is all run from the starter battery.

My plan is to run a single feed to a small fuse box attached to the dash. From this will be a ciggi lighter (for a tv, phone charging, sat nav etc - I don't smoke!) and the head unit itself.

I'm planning on fitting an amp at some point but assume it would be best to run seperate 12v cables for that when i've got one.

I'm unsure of what gauge / ampage cable to buy to run from the leisure battery itself to the newly fitted fuse box under the dash.
My other van uses 240v fairly heavy duty cable for a similar setup!

Also, for future planning - when i eventually fit a sub and amp would the 12v power be best provided from the starter battery or leisure battery?
Also (again!), should i add a switch to the dash for the head unit itself - to give me the option of switching from starter or leisure battery for power? I'm thinking i'll wire the 12v continuous backup supply to the starter battery, but the main power input from the leisure. I'm more likely to take the leisure battery out of the van to charge it... and don't want to have to reset the stereo settings each time!
I'm a little unsure how the leisure battery earths, I need to grab the multimeter and take a look. Not sure if it's negative earth to body - or will that just be the starter battery (can both battery's use the body as an earth?)

Thanks for your advice & sorry for (without a clue) ramblings!