I'm attaching a schematic that might work for you. I picked an LM317 for the regulator because Maplin doesn't seem to have the LM7806, but I don't know what your resources are. If you select a different relay, it needs to handle 10 amps of contact current and the coil should draw less than 100ma at 12v. The 12 AWG refers to the minimum wire size you need for the wire that interconnects the battery, the relay contacts, and the spotlight. The size of the rest of the wires is not critical.