Hmmmm, paranoia was starting to set in! So a quick chat with our developer guru to find out why we are showing the following error for our sites:
For any technical minded members, or in case you are also showing the same view port error.
Apparently we used to have the mobile site set to "width device-width” but it didn’t size correctly on larger mobile screens, so it was set to a fixed width which works for everyone, and Google is quite happy with.
To confirm the bugginess of the Bing tool, it reports the site as not being mobile friendly in the tool, then in the search results, guess what...

It marks it as Mobile friendly, in the same way as Google does.
Just tested again while I was writing this post, and for some bizarre reason the Bing tool seems to have gone all Italian on me! 

B- Bing, must try harder!