Stephen,
I don’t have any I can help you with, but I do have a solution for getting the “yellow” out of old decals if you’re not in a big hurry. Place the decal in a clear waterproof bag, and tape it the inside of a window that gets sun for a couple of weeks or so. The UV rays will remove the yellowing. It may take longer for really yellowed decals, and you need to check the decals occasionally to make sure other colors like red or blue are not fading (this usually takes a long time— several months). I’ve saved lots of decals this way. My latest was a 1962 issue of Revell decals for their Atlas booster and launch pad. Within three weeks, the sun had removed over 50 years of yellowing! If you live in a climate like Arizona, you need to make sure the decals are not in direct contact with hot glass, but since you’re in the U.K., and I live in the US NW, I doubt you and I have that kind of problem, especially this time of year!
VR, Russ