Red Kites seem to have generally preferred to fill in their core range before spreading. I remember after going to see the Oxfordshire Ibechiff at Great Tew in 2001 deciding to nip down the M40 to see how easy it was to see the Chilterns kites, because at that time they were still quite scarce in Bedfordshire and we wondered when they were going to arrive en masse.
We had the first pair of kites over the motorway just before we got to the Chiltern scarp, and after turning off we stopped on a hilltop, scanned around, and counted 20-odd. Obviously they found the habitat to their liking and found no need to spread out.
Nowadays we probably have numbers like that in Beds - it’s difficult to drive more than a few miles in much of the county without seeing one, so they’re on their way to Notts.
For some reason the reintroduction in north-east Northants near Peterborough never seemed to do as well as the Chiltern one - more rural, more persecution? It’s possible some of the early north Beds records may have come from there, but more numerous birds in the south and centre of the county probably came from the Chilterns. If the Northants birds had done better they might have made it to Notts quicker.