And just in case I need them, where can I get the APM on board 500mah fuses from in the UK ?
The best way would be to buy a BEC, as this will prevent brownouts on the APM during hefty servoaction.
I've go for a 6v instead of 5v, as this gives you a quicker servo response. (Just check that your servos can handle it)
Personally I'd also get a 5v BEC for powering just the APM also, then you are not reliant on power from any of your ESC's. I've heard stories of brownouts during hard throttle, which would send your copter plummeting towards the ground.