Thankyou everyone for your help, I have now bought everything I need. I went with a cool little UBEC I found on ebay- a few people on RCM use them and report good things about them, so I went that route:
6v/3amp switching bec- I will remove the red connector and solder it my esc battery leads.
I also found a cool little lvc type device, which has an LED to warn of low voltage on your batteries- for 4s use it detects if the voltage drops below 12v (3v per cell) and the led comes on. I will add a little buzzer I have to the led leads so I have an audible warning too:
All I have to do now is sit back and wait for the goods to arrive- 10-14 days for everything to turn up.