Yeah, I did various combinations of create new project and disabling and uninstalling the Arduino extension. Best I could get was a mix of Arduino and Particle icons in the top right/lower right panels, but still had compile errors.
I have now uninstalled/reinstalled VS Code, reinstalled Particle extension and uninstalled Arduino extension - this did not fix it.
It is fixed now, just not sure why. Seems to be linked to some weird file structure I had. This folder structure:
Resulted in this workbench structure:
The .vscode ‘folder’ at the higher level had a c_cpp_properties.json file in it (now deleted) that I I shouldn’t have according to Error: "Configure your Intellisense settings to help find missing headers" (was: Arduino extension required?)
After deleting the c_cpp_properties.json file and eliminating the higher level folder, all seems to be okay. Touch wood.