I'm not sure if these will be of any help but Lotus Food Market, Parkson and Pines are other places that we list in our Shanghai guide as places for finding imported goods.
Buy it in Taobao. I buy many imported products, either by people living in Shenzhen who buy it in HK and ship it to you, or people with relatives in other countries and they just buy them over there and ship it to you.
I checked MARMITE in taobao and many people has it. Just ask some colleague or friend who can read chinese to choose the right seller for you (or just check the symbols and stuff, is not that difficult actually, just at the beginning!)
Good luck!
