Thank you for the latest information. This is where the confusion begins. The manufacturer, if my translation can be believed, told me to remove the paper backing after placing the decal in position and pressing it down. The backing paper should be dampened with water to assist this. Then proceed with the rest of the process.
I found a glass bottle of Rowney 800 picture varnish that I bought decades ago [it cost 16p !], when at the time I had an inspiration to do oil painting as a hobby. I never opened it and now, in the bottle it looks OK, clear not cloudy.
I tried a small amount of this varnish on the decal sheet border and it dried virtually immediately or just soaked in, so pretty useless for this situation.
I'll search on.