Christopher,
Yes that seems very likely and yes Wl Mill on the Ryton Parish registers is Winlaton Mill.
I've just read some of your detailed history of the Spencer Family essay on your Family Tree on Ancestry (found when searching for Philip Spencer), really fascinating. My wifes GGGGGF and his three of his sons, including her GGGGF all lived in Newburn in the 1840's and 50's and worked as Spring Makers, File Makers and File Hardeners so presumably worked in the Spencer Factory. All came from Yorkshire, how they ended up in Newburn is a mystery.
A few snippets that aren't on your essay that may be of interest, from things I noted when looking for a connection between John and Philip :
Regarding Johns oldest son William :
Burial at Newburn. 29 April 1839. William Spencer, age 25, File Maker.
Newcastle Courant. Fri 3 May 1839.
Died. The 28th ult. at Newburn, age 25, much respected, William, eldest son of Mr. John Spencer of the firm of Messrs J. and W. Spencer, Steel File and Spring Manufacturers. Mr. S. arrived from off a journey only on the previous Friday, was attacked with an inflammation the same evening and died on the Sunday morning.
Regarding Johns brother William, I was hoping to find a connection to Philip from him :
Baptism at Newburn. 22 Feb 1824. Jane, daughter of William and Bridget Spencer of Newburn. File Manufacturer.
Burial at Newcastle St. Nicholas. 20 Feb 1829. Bridget Spencer, age 47, of Scotch Wood.
Newcastle Courant. Sat 28 Feb 1829.
On Monday last at Scotch Wood, Mrs Spencer, wife of Mr. William Spencer, and daughter of the late Mr. Joseph Marshall of this town.
Marriage at Newcastle St. John. 2 Feb 1834, William Spencer of Newburn, and Esther Gardiner of this Parish. By License. Groom a Widower, Bride a Spinster.
Newcastle Courant Fri 25 Oct 1839,
The 26th inst, in Blenheim St. in this town, aged 56, Mr William Spencer, much respected, of the firm of John and William Spencer, manufacturers.
1841 census. Blenheim St., Newcastle,
Esther Spencer, 45, Ind., N,
William Spencer, 15, App. Patent Axle Maker, N,
HO107/824/14/3.
Johns wife Barbara burial :
Newcastle Courant. Fri Feb 8th 1861.
At Lemington Hall on the 5th inst., aged 78, much respected and deeply regretted, Barbara, wife of John Spencer Esq.
Hope this is useful,
Alan.