Thanks again to Alan (ajgriff) for the link to Harrison & Harrison where I purchased some 19mm Brass Discs - Thickness:0.9mm (UK 20 gauge).
Don't look too out of place in my opinion: -
I appreciate they are flat and not slightly concave like the originals but they are
"an acceptable compromise".
Once I get them properly polished and give them a coat of Rustins metal lacquer they will look even better.