Could I fly from a) Siem Reap to Shanghai via Bangkok and the return Shanghai to Bangkok on this vis

Bangkok > Shanghai > Bangkok is not a transit so its not eligible for the 24/144 hour transit without visa. The first landing after China needs to be a different country than the one you entered from. If you add a landing in Hong Kong on the way to Shanghai or on the way back from Shanghai you can then take advantage.

Dear sir
So if i want to fly bangkok shanghai bangkok i have to ask for a visa online?
Many thanks

You cannot apply online. You can apply for that from the China embassy or consulate general in person.