Dave Plowman wrote:

The current escutcheon on my front door mortice lock has a round chunky
cast brass cover - hinged at the top. I'm changing the door furniture to
brushed stainless steel and have found everything else apart from that.
All I can find with a cover are those rather cheap looking pressed from
sheet domed ones.

Presume you've searched ironmongerydirect?

How about this?

https://homebits.co.uk/acatalog/ARKITUR-design-hardware-stainless-steel-round-lever-key-hole-swing-cover.html#SID=6927