OK, I am getting a better idea so far on this.
The 4 pin and 8 pin share the same advance curves. the original 8 pin, no longer available new or used does have 12 separate curves built into it though which were modifiable by swapping over a plug in resistor on the module itself ,but the 420 type doesn't ,having only 6 different curves which are accessed by the adjustable pot in the passenger footwell. I've not had the need to touch those. The extra pins in the original unit are for the two knock sensors.
I was thinking the timing might possibly be able to be tweaked so I will put the light on it and check the operation as it goes from idle to part throttle. But that sounds the most plausible reason so far.
As an aside,a mate has removed the KEjet entirely and fitted the Kjet from a 380 to a spare 560 to see how the system likes it.

And, there is no lambda control or oxysensor on this version of 560 either.
Again many thanks!!!!