@Kammee
I think a lot can be achieved with correct body language, tone of voice etc I did find a good trainer who she seemed to walk nicely on the lead with just through this ladies mannerisms. She gently tugged lead when she went out of heel position and it seemed to work.
The issue with this type of training, is as soon as a "gentle tug" stops working, the owners' instinct is to tug harder until it does work - meaning you can cause real damage to your dogs' throat and windpipe.
Yes, body language and tone of voice are important, but dogs thrive off praise and rewards. You want them to do as you ask because they want to please you, not because they're scared you're going to hurt them by pulling them about by the neck.
The idea with treats is that, yes, you use them a LOT when they're puppies but over time (as in, over a year or so) you phase them out, but only once the dog is responding 99% of the time with treats.
So, my beagle will sit, stay and do various other tricks without treats now (aged three) so I can use praise for those things. However, due to his breed, his recall needs to be constantly reinforced, so every time I call him back, he's rewarded with sausage.
It's not bribery - it's training and keeping him safe and under control. If he knew there was a chance I wouldn't give him anything, he'd stop coming back at all.
Heel training with her will be similar - breeds like spaniels aren't designed to be kept on leads, they're bred to run off into undergrowth and flush out birds. So being on lead is unnatural to her. So, for now, you will need to use lots of treats to encourage her behaviour - this is going to be especially tough as she's in her teenage years, but you just need to persist. Personally I would use a harness with two D-rings (one on the chest, one on the back) and a double-ended lead so that you have as much control over her as possible while you're working on her lead manners.