I don't buy fabric conditioner, haven't for years and years, but when I did a search for "removing hideous decoupage" I was told time and again that this was the product to go for.
So I nipped to the co-op and picked up a bottle, mixed four capfuls with hot water in a spray bottle and set to it.
Just like removing wallpaper, I put some slits into the layers of paper then sprayed. Left for a couple of minutes then attacked it with my stripping knife.
It works! It did take a while, but I was listening to London v Widnes so it didn't matter. Spraying and scraping started to reveal the shelf
About an hour later it was done!