it's British-speak: you have to allow the decal to fully dry before you over-coat it with (more) varnish. You do want to clear-coat the top of the decal with the same high quality varnish used to adhere it (and a good soft brush) for protection but don't do it immediately after you've adhered it or you'll ruin it.
I'd wait a couple days (not just 24 hours) to be extra cautious.