How do I remove cast reflective vinyl that has been stuck on a British Gas van for 10 years?

Warm the surface of the vehicle, with a heat gun or hair dryer ( put it in a bodyshop oven if possible) this will soften the adhesive, then apply RAPID REMOVER , then using the QR tool start scraping the corner of the reflective vinyl decal and get the plastic scraper blade under the graphic, between the decal and the vehicle bodywork and with liberal use of Rapid Remover the adhesive should yield and then it is just a matter of removing the residue from the bodywork.