Thanks for all the comments and the various methods. Here's the one I've successfully tested then used to secure the decals for a great finish and any amount of rough handling. I guess everyone has found something that works for them, this is mine - if it's useful to anyone else coming new to this.
1) Use Zero Paints model car polish to polish up the area the decal is going on to a mirror finish. There's no point wasting polish or effort so only the area I want to decal is polished. The Railmatch Enamel is hard wearing paint and will stand up to this polishing no problem.
2) Apply the decal and leave overnight to dry. Press hard on the decal after appication to squeeze out air, water, etc. After it's dry, spray over several mist coats of Mr Hobby Flat matt, then a heavier coat - all from about 20cm away. This returns all the paintwork to a uniform matt, whether it was polished or not. No decal crinkling. Job done.