Hi Debbie
You are right to be prepared - I did this trip at the start of October and weather.com was quoting an average of 23-25 degrees, but that didn't account for the cloud cover and the horrendous pollution - it was absolutely freezing!
You jest about thermals but, believe it or not, I always travel with a couple of pairs for night time long haul flights and I ended up wearing them for 75% of the trip!
Don't be put off though. For starters, I have heard that a lot of the pollution has cleared since my trip because they shut down all the factories in preparation for Obama and Putin's recent visit (they did the same thing a month before the Beijing Olympics). Take loads of layers and you will be fine. Also, take loads of toilet paper and antibacterial hand wipes because the 'facilities' for ladies are, how shall I put this.....,'variable'.......
It is an amazing trip and you will have a ball - let me know if you have any questions and I'll be happy to help. One special recommendation, whether you are an animal lover or not especially, is to do the 'Panda Keeper Experience' in Chengdu. It costs 2000 Yuan (£200ish and they need cash) but it is truly worth every penny. Those of us who did it left with our hands shaking and a goofy grin on our faces that didn't fade for days. One of those 'once-in-lifetime' moments that you will remember forever.
Have a super time
Mandy