If I remember right, forgive me but MB keep changing things, but AFAIK the GCB or whatever they call it today is £250 for the 3 completed years and generally is paid, from my past payments, within a week of collecting the new car. Not sure what they allow for the extended years but my last car was returned at just over 4 years and if I remember right I got an extra £100.
Again things keep changing the past few years but going back to Covid years the dealers were, with MB permission, I believe taking the bonus off the AP but not sure they would do that now.