Tow bar wiring on a 59 plate bluemotion polo hatchback + cutting bumper?

I have bought a tow bar & single electrics for towing a trailer. I am ok in wiring the socket & maybe the lights to the existing wiring loom in the boot, but I have read somewhere I may need a relay. I also wish the light on the dash to work instead of an annoying buzzer, again, is it a case of plugging the trailer into the socket and the car will sense the extra lights and make the dash lamp flash.
Also will I have to cut a hole in the bumper as I have the detatchable ball type and if I need to do this can I get a blanking plug when I am not towing to cover the hole up


Thanks,

Rob