I did a search on births in the GRO Index on findmypast.co.uk for Ninn in the UK from 1950 to 2006 - not just Kent (I restricted it to exact spelling - no variants), and only got the following:
NINN Linda D England Kent, Tonbridge 1950 VIEW
NINN Rosemary England Kent, Tonbridge 1952 VIEW
NINN Douglas J England Kent, Ashford 1953 VIEW
NINN Nicholas L England Kent, Ashford 1961 VIEW
NINN Gillian D England Kent, Ashford 1964 VIEW
NINN Christopher Geofrey England Kent, Ashford 1978 VIEW
NINN Vincent James England Kent, Tonbridge 1978 VIEW
NINN Maria Christina England Kent, Ashford 1981 VIEW
NINN Abigail Holly England Sussex, Hastings & Rother 2004
Note how they are still concentrated in Kent, and particularly in Ashford which is adjacent to Great Chart.
It does appear that the name is perilously close to extinction!
When you do the search allowing variants, there are other names such as Ninnes, Nind, Nine, Nines, etc and they are all scattered over the UK.
However, I feel that the surname Ninn is unique to the survivors from Ninne House Manor in Great Chart, and which was spelt Nynne prior to 1500, and possibly was just Nin before that - that is my hypothesis anyway.