got your e mail mike, and i've amended my post regarding email address
your uk sp370 diagram makes more sense regarding the smaller charging diagram i posted earlier and to my first post on the problem. my dr 400 is the same as you 370 diagram with the exception that the dr is cdi not points.
the resistor is placed in the diagram just above the magneto, and mine doesn't have the voltage regulator shown on the end of the grey wire.
so to speculate again, the two feeds off the magneto need to reach ground at some point to obtain 6v. if your lights are off, then only the rectified circuit will ground giving you 12v. to combat this, the resistor is switched in circuit to pull the voltage to 6v. this isn't a 'regulator', only a resistor. if the resistor fails open circuit then your battery will not charge when lights are off. if the resistor fails short circuit, then your battery will cook with 12v or the rectifier will blow.
with your lights on, both circuits are grounded, so there is no need for the resistor. however only the headlight and instrument lights are ac, so if they blow or become corroded they wont reach ground, so 12v goes through the remaining charge circuit, this time with the resistor switched out and cooking your battery or blowing the rectifier.
with a damaged rectifier or knackered battery your lights will blow every time they are turned on as there is no ground on the charge circuit so the lighting circuit goes to 12v.
i think what is needed is a belt and braces approach like you said mike.
if you fit one of thesehttp://cgi.ebay.co.uk/BSA-BANTAM-D1-D3- ... 53db8eb67b
onto the end of that grey wire, as per the usa diagram i posted then i think all areas would be covered.
assuming your battery is ok, and your mag resistor is ok and all terminals are free from corrosion, then if your main beam should blow (possibley due to vibration) then the excess voltage that would appear would be dissipated away preventing voltage surges and damage to the rest of the system.
for info, my resistor reading is 5.6 ohms
the rectifier is 1.5 Mohms one way, open circuit the other.
as has benn posted before, it's a delicate circuit and relies on everything in tip top condition.
does this sound feasible
unfortunately i live in the north east, the otherside of the country.