SportCityRider,
For some reason HRG was familiar to me as I seem to remember seeing HRG at Goodwood a few years back.
HRG in Wikipedia brings up the details of the company thus:
"HRG (HRG Engineering Company Ltd) was a British car manufacturer based in Tolworth, Surrey, and founded by E.A. Halford, Guy Robins and Henry Ronald Godfrey in 1936. It took its name from the first letter of their surnames."
It's handy that Mr Godfrey's initials are the same as well. Maybe he was the majority shareholder?
Now my experience of the gospel according to Wikinonsense is that it is always right, so we must be in the right ball park.
There seems to be quite a following, though it appears that more cars were modified than bikes.
Perhaps someone with more knowledge of Brit engines and frames can help you with what it is based on (I'm thinking Norton Featherbed frame and a Triumph twin as it was probably the best combination).
Contact mailto:hrgsec@pcmitchell.com who may be able to point you in a more fruitful direction for info on bikes.
Tony B.