well some people are mad over authenticity when it comes to older cars
Controlling the field coil will not decrease the power
I think you'll find that that's how it works, both originally, and as you're doing it.
the dynamo needs regulating at all times or you can end up with 40 volts (I managed 18 before pulling the plug on it and it looked eager to keep rising)
How?, the battery will stop the voltage rising that far, just as it does on the original regulator, and just as it does for an alternator.
I can set the current limit in the software and it will run to that, so if I have a temp sensor and related algorithm to increse the amperage figure when the weather is cold I can allow more power to be generated in winter but keep it to normal levels in the summer so as to not burn anything out.
You seem a bit confused about what you're doing? - as I said you can't increase the output, only decrease it - and that's what the original regulator did. No need, and no point, in monitoring ambient temperature - you charge the battery until it's full, then drop charging current to a low level to keep it topped up - same winter or summer.
I suppose I could just replicate the "burst fire" control of the original and run at a high clock speed, at the end of the day it still results in a crude PWM but with a drift in frequency, in his case this was 70-150 Hz hence my choice of frequency. in any case once built I can use both versions of control, thats the beauty of the uC method I get to play with variables without unsoldering stuff all the time, or redesigning the whole project.
as it happens i got a more steady regulation than the original reg, so I did something right
Why did you want a more steady regulation?, what advantage does it give?.