Hello Damien!
Fantastic collection of photos of a very worthwhile project.
I can't add anything to what John has said about it's history, but I do recall seeing a white Mosquito at Stoneleigh around 1985 parked next to a Stimson Scorcher.
Have you seen this page:
http://www.3wheelers.com/mosquito.html
Not much there, but interesting.
By the way.
Alloy with circles - I've always called 'machine turned'
and 'Primer' sounds like the right word to me!!!
Cheers,
Ferg