Don't know why this has just popped into my head. Will have to ask someone to follow me this weekend. But whe acc is on and it's slows the car down quickly, does it put the brake lights on . Also does it  put them on if braking slowly. I get the feeling I'm asking a daft question as they would come on for safety reasons! But as I can't actually see them

If the car brakes then the brake lights come on.

 

Not sure what you mean by brake slowly - if the car brakes the brake lights come on, if the car decelerates as if you were to lift you foot off the accelerator, then the brake light won't come on because it hasn't used the brakes.