I can't comment on Which's results but we publish Real MPG based on user submissions here: www.honestjohn.co.uk/real-mpg

All new cars are subject to official WLTP fuel economy tests which are designed to be more representative of real-world driving than the old NEDC tests. They're not perfect but they do allow you to compare cars like-for-like. The issue with plug-in hybrids is that they're reliant on regular charging to get the best from them. The majority of PHEVs, however, are leased by company car drivers for their BIK tax savings. If you're a company car driver covering hundreds of miles a day and not charging the car, a PHEV will be woefully inefficient. If, however, you charge the car overnight and rarely travel more than 20 or 30 miles in one go, you'll see incredible fuel economy as the petrol engine will hardly ever need to run.