You will need a diode to stop the leisure battery feed back feeding the canbus.
I put a feed in from my leisure battery via a switch. The feed went into the fuse 19 feed (canbus), but when it was on the voltage flowing back down caused the central locking to stop working. Put an inline diode in the fuse 19 feed which stops the back flow.
Run a feed for your switched / ignition live from fuse 19 (brown and green wire I think) from the back of the fuse box. That will give you the functionality of ignition live and your stereo will work until you take the ignition key out (there isn't a normal ignition live that will give you this like other cars, that's where the CANBUS gets involved).
So normal permanent live into stereo. Ignition live from fuse 19, then a separate live from the leisure battery via a switch into the ignition live (same as the fuse 19 feed). Put an inline diode (the right way) on the feed from fuse 19. This will stop the feed from your leisure battery backflowing to the canbus when you power the stereo just from the leisure battery.
got mine from Upspex for 20 something quid, reasonable price and easy to instal by the time you have bought all te bits, assuming you dont have them already including shrink and Fabric Tape you probably wouldnt save that much money anyway IMO and you get clear instructions for fitting