While out on his walk last night my year old dog was nasty to another.
He was fine on his walk, stopped and played with a little yorkie bitch he often bumps into.  he then bumped into someoneone walking a collie x bitch who he has met before and got along with fine, they got nose to nose and all of a sudden he went mad, snarling and trying to get at her! he has NEVER done this to any dog before, he meets all dogs happily and is always friendly. Both dogs were on leads so easily pulled apart. The owner of the bitch was surprised as well as she knows my dog to be friendly but she said her bitch was a bit "off" lately and was wondering if she was having a phantom pregnancy (didn't ask why she thought this) but she said she was acting odd lately and keeps licking at herself and always wanting cuddles.

Surely that wouldn't be a reason for my dog to want to attack her? I have no idea why he would out of the blue act like this? any ideas?

Did you notice what body language the other dog was giving yours? there may of been something and you didn't notice then your dog reacted to it.
I've got a few dogs i meet when i go out and when their on the lead they can be a bit funny but off the lead they play happily together.
Maybe the bitch was giving of a different smell if she was having a phantom pregnancy but i don't really know alot about those so thats a guess.
hopefully someone will be able to help more but i do think dogs can be like humans sometimes and have off days, so maybe your lad just wasn't in the mood last night.

What breed do you have?

There are certain breeds of dogs that do not react well together.  I have done a lot of research into dog breeds and reactions of those towards each other and my findings are quite rediculous.