UK: Flying Squirrel or Sugar Glider?

Some sort of small gliding mammal has been spotted in Wimbledon Common. From the news:

"So far educated guesses on website Wild About Britain have included the Australasian sugar glider, the North American flying squirrel or another species of possum." ...
"Wandsworth's Kolin Barnz, who spotted the beast while on the commen, said: 'It was squirrel-like but its face looked more mouse-like, with long whiskers, black eyes and small ears.
"'As it jumped between trees, flaps of skin stretched between its front and back legs and it gilded to the next tree. I couldn't believe my eyes.' How the animal got to its new home is anyone's guess, but it is possible it could be someone's escaped pet."