LAB:
That P800 appeard almost 2 years before I joined N-V (in May 1967). Externally, the DOHC engine I saw looked just like the Atlas/Commando 750 externally. I never saw it in a frame, so I don't know if it would've been slanted forwards.
Particularly, the crankcase was standard height, which the P800 doesn't appear to be.
I think the P800 was seriously wounded with the take-over of Norton by AMC and the Managanese Bronze/Villiers take-over was probably the death blow.
There were a lot of very poorly conceived and managed programs in the various UK motorcycle companies around that time. One that sticks in my mind was an attemtp by Ariel to make a "Super Leader", with a 650cc four-cylinder boxer configuration. At the time the 250 2-stroke twin put out about 26 horsepower. The prototype 600 Ariel bench testerd at 22 hp.
I think the concentration on cheap high-performance transport for the blue-collar UK market blinded management to the growing high-technology "toy" market in the US.
The reason I heard for the demise of the DOHC engine was that it would have cost too much. I wanted to do a derivative of the Commando that would've been an in-line 4-cylinder 1.0 Liter, DOHC, water-cooled, electric start engine with a 6-speed transmission and shaft drive. My concept was to offer a clean "stripper", essentially looking like roadster Commando and a full-up tourer, with a factory installed fairing, panniers, etc. I must admit that I wasn't brave enough to suggest a four-banger lying sideways (BMW) but I did try to get automotive-style carburation, not four Amals.
Except for my engine being an in-line 4 rather than a boxer, my bike looked a lot like the early Gold Wing.
Unfortunately, our manufacturing technology was more 1890's than 1970's, so I don't think we could have made it without some massive investment in manufacturing and engineering, and the UK industrial and financial climate in the late 1960's just didn't support those kind of plans.
Pity - what we could have done.