I also did a stopover in PVG on a trip to Japan earlier this year. You will need to take a printout of your onward itinerary to Japan with you, because the immigration officer at PVG will want to see that as proof of onward travel within 144 hours.

Signage for TWOV at PVG is not great. As you walk into the immigration hall you will see a long line of desks, where people will be queuing up. You should bypass the queues and go to the desks for 144 hour visa, which are at the far left end. There you will need to fill out a short form before approaching the immigration officer with your passport and onward travel details.

At BA the check-in at LHR I was asked if I had a visa for China, and I just said that I would be entering on the 144 hour transit visa.