Hey Matt, being a time served spark and a gutter sucker... maybe i can help clear this up?
regardless of what anybody says including omni-pole you can quite comfortably get away with 2.5 mm cable upto 50 metres, however depending on wether you are 110v or 240 4mm would be the safer bet at 100 metres but to be honest you would not experience any voltage drop at 100 metres using either...
dont wory about over heating cable ( residential properties can run 120 metres of 2.5 without a single insulation or voltage drop problem) just be aware of external factors, ie cold, wet etc so my advice and what i use is 4mm artic cable split into 2 sections so i only use 50 m when poss, with another 50 metres to add if ness.
james