Thank mate,

I'd found those measurements and even a part number (E830607576).
So, I took a look on the web and found said washer is a rarity.
The good news is that, when working on the car this afternoon,
the missing washer showed up! It was lurking on the scuttle
panel under the rear end of the bonnet. It's slightly rusty
but I have a grit blast cabinet so the washer will be minted
up and reused.
I've alse deciding to make life easier by buying new
front struts and springs. My kit car's bodywork can't
rust but teh hardware below decks certainly has.