I have a flight booked for the 28th December flying to Vietnam, I'm going to be lone travelling which at times feels a bit scary. After Vietnam I'm planning on going to Cambodia, I've heard it's a bit unsafe there do you think it's best I book a planned tour whilst there, and is it best to book it before I go or can I do it when I'm over there? 
Travelling alone SEA
Cambodia is perfectly safe to travel in as long as your sensible, I'm heading to Thailand, Vietnam and Cambodia, alone from March 2014. you should look at STA travel they have hop on hop off bus passes for Asia including routes that follow the main sights in Cambodia.
Blue Elephant
Do it yourself. You'll be safer in Cambodia than in Bristol, UK.
Besides, there are more frequent buses in Cambodia than you presumably have at home. And they're good quality too. There are lots of others travelling Vietnam and Cambodia. Just make sure you find the right guesthouses to meet them.