I'm confused: I was told I was getting warmer when I identified it as a London-Edinburgh 40/50 Rolls-Royce
When I suggested you mean it's a Royce, but not a 40/50 (aka inaccurately as Silver Ghost) I was told "not a Rolls not a Royce"
(perhaps the shorthand "Royce" for Rolls-Royce confused you - it's how they referred to the company themselves, as did we who had dealings with them)
Someone said Invicta and you said go south: Derby is north of Cobham
So
it is a Rolls-Royce
it is a 40/50 (often generically called "Silver Ghost", though strictly that was one car)
it is a London-Edinburgh (or later) (cantilever rear springs)
probably about 1913 from the chassis number, but never Dated by the Veteran Car Club of GB
The classicdriver link didn't work for me, by the way