Deano,
Before you decal, tape the decal sheet to a window that gets a lot of sunlight (but not humidity) for a while - checking often - it might take a week or two - and the light will slowly fade the yellowing decal carrier. Then just use the decals.
Alternatively, use a new no.11 blade to carefully trim around the image, effectively detaching the decal from the clear carrier surround. Cut just deep enough to cut through the decal, not the paper, as it's easy to damage the decal. Then just dip the decals in water, and using your knife or some tweezers, remove the clear waste from around the decal and apply the image as normal.
Cheers,
Lee Tree