I would say it's a hybrid of Hydrangea serrata, note how the leaf venation is more sparse and different to others. The sepals on yours are serrated, I haven't found one like that yet.
https://www.google.co.uk/searc...
https://www.google.co.uk/searc...
I found one, a collection by plant hunters who have a nursery in Wales who have met up with Dan Hinckley, another plant hunter, they collect in places around China, Taiwan, Cambodia etc. Dan Hinckley's business was taken over a few years ago if I recall correctly, but I think he was in California.
Hydrangea aff. zhewanensis - MF93117
https://www.mailorder.crug-far...
It's possible it came from Dan Hinckley.
More info about Hydrangeas..
http://www.hydrangeakwekerij.b...
A search for Hydrangea zhewanensis brings up Hydrangea chinensis, I think this is close.
http://america.pink/hydrangea-...
https://www.google.co.uk/searc...
Dan Hinckley!
Introduced to N. America by Dan Hinckley in 2008.
Originates from southern Sichuan Province, China.
http://www.heritagehydrangeas....
http://www.monrovia.com/plant-...
