Many thanks indeed for all your comments!
Have decided to go for it and have asked my BMW Main dealer to add on to the spec. There is no response as yet, but should think it poss as it's for a week 32 build, commencing 6th Aug, for Sept delivery.
The 'Active Cruise Control with 'Stop and Go' function' at £620 is in the latest BMW configurator for the 320D M-Sport, which was updated a couple of days ago.
To have this function it would seem the motor requires the Auto gearbox and the Black panel display,full. Which I have speced!
Will keep you advised!