On that note, if I use the clicker outside of training sessions do I HAVE to give a food/toy reward after (as thats what turns dogs on to the clicker) or will praise do fine?
If you're using the clicker to mark behavior you're IN a training session, whether you think you are or not.
And yes - to use a marker correctly, a reward ALWAYS follows. The reward doesn't have to be a treat, it can be a toy, or a real life reward such as access to anything the dog values. "Verbal rewards" aren't really rewards, per se.
The reason clickers (or verbal markers such as "yes!") work so well is that the sound signals that the dog did something rewardable, hence the reward. So if you mark behavior but DON'T reward, then the sound stops being meaningful to the dog.
Praise is nice, and I use it profusely, but it's not usually inherently rewarding in the way that food is. Food is what's called a "primary reinforcer" - you don't have to teach your dog to value it because he needs food to survive. My dogs love it when I tell them how wonderful they are with great enthusiasm, because they've learned that when I'm happy with them (to borrow a phrase from Jean Donaldson) "good things happen for dogs". But if I had continually praised them without good things ever being paired with it, they'd just be meaningless words to my dogs.
I'll use a variety of words such as "good!", "great job!", "nice!", and "excellent!" if I don't want to give a reward, but I do want to let them know I'm pleased with their behavior. But the marker always means a treat will follow.