DON'T. Really, don't. I really do know the frustration that goes along with it. If you think it might be the quality of the ones that you have to work with, then see what you can do about getting other brands a little at a time.
If you come up with even just a couple things that are suitable, you're ahead of the game. Do you know at what % the ones you've worked with tasted best in just a single mix? Take notes on that for each one. Once you find that base number, play around with adding another flavor to it, that would compliment it, but not in an equal amount, just a bit smaller.
Even vaping unflavored keeps some very happy. Not me, I want flavor, LOL.
Brands you might find easily, could be FlavourArt, and maybe Inawera. Even TFA, (the Flavor Apprentice) is one that I see mentioned in the UK. Each of those brands make some VERY good flavorings. But, not knowing anything about the ones you do have, it's hard for me to pin point what may specifically help.
The best thing I can think of is to make small samples of single flavors, starting with a lower percentage, taste it, leave it overnight to calm, taste it again. Too weak? Add a touch more. Take notes til you find the spot you like. Easier to add to, than have to dilute. Patience is the tough part starting out. It certainly was for me.
Try out recipes. Doing that SAVED me, and it taught me about flavoring.
If you haven't asked about that brand, do so. They might be able to help you narrow it down.