Gavin, I've used after market decals which were laser printed onto rather thick backing and like your decal, they would not stick using normal materials and methods. I managed to use them by mixing a blob of PVA and a small amount of water and used the sticky mixture to adhere the decal. You'll need to clean up the surrounding area pretty quickly though once the decal is positioned, using a damp cloth.
You could try an acrylic varnish instead of the decal solution as an adhesive, I'm sure there used to be a video using Humbrol Clear on Youtube to do this.