So having installed the physical bits of the winch, I now need a split charge system to run the two batteries. The winch is connected to the second battery in the boot and the main battery runs the normal car systems. A split charge relay connects the two batteries together, but only when the charging situation is good. The relay takes a feed from the alternator and when it is "good" it connects the batteries together. There are many types of split charge relays around but many are only suitable for low current applications such as caravan systems.
The relay below is rated at 200Amps continuous which means it can cope with high currents between the two battery systems when they are connected together.