Here is a collection of quotes on the topic about when Clumbers first appeared at Clumber Park.
JOHN MANSELL, GAMEKEEPER TO THIS GRACE THE DUKE OF NEWCASTLE, HOUGHTON KENNELS, OLLERTON, NOTTS in THE FIELD 1870
I do not know the exact date of the spaniels being introduced into England, but it must have almost as soon as the duke inherited the title (1768), as the Mansell in the picture was my great grandfather, and they first came in my great, great grandfather’s time.
RAWDON B LEE MODERN DOGS, SPORTING DIVISION 4EDN VOL II C 1906 / 1917
When the first introduction took place is not exactly known, but it was probably between the years 1720 and 1730, when Captain Robert Spencer brought specimens over from France, probably having obtained them from the Duc de Nouailles. They were kept at Althorpe Hall some time before Captain Spencer presented a brace to the then Duke of Newcastle.
TYZACK’S ANNUAL circa 1910
The first given to Henry Clinton afterwards Duke of Newcastle, by the Duc de Noailles, and William Mansell, about 1764.
SABIN’S ILLUSTRATED CATALOGUE OF ENGLISH SPORTING PRINTS 1933
The Duke, when in France about the year 1775 received a gift from the Duc de Noailles, several of this breed of spaniel, which he took home with him to Clumber.
RAE FURNESS THE GREAT HOMES OF THE DUKERIES CLUMBER SPANIEL CLUB YEARBOOK 1983
The seventh Duke of Portland always contended that the Clumber Spaniel should really have been either the Portland or Welbeck Spaniel, as it was to his ancestors that the Duc de Noialles gave his white Spaniels.