https://www.carbonda.com/handlebar/chb-1056.html

I'm after something similar (and as trustworthy) in 120 x 400.
Any links appreciated.

Carbonda is a top tier carbon producer. You can trust

Correct, but they don't have a 120 x 400 mm integrated stem and bar handlebar....

https://www.lightcarbon.com/super-light-carbon-road-integrated-aero-handlebar_p246.html

I can recommend bigrock skyline 2 handlebars. Definitely a premium product in my eyes