FYI - for others with a similar problem.
It was just a matter of overriding the Bootstrap Embed class with a percentage = to the video ratio (as the video was a non standard size)
You can work out by dividing the heigh by the width in px.
In this case… 1335px / 3584px = 0.37248. So 37.25%
Normally you would make a new embed class, but if you want to use the Blocs video Bric you can override the Bootstrap default class…
<style>
.embed-responsive-16by9::before{padding-top:37.25%!important}
</style>