I have found few references to a Mary Cumpston of the right age in the County of Durham. A trawl of appropriate resources has netted very few facts.
1780 Longbenton P000341 C00034-2 Cumson Mary daughter of Cuthbert and Dorothy christened IGI 16 Apr 1780
1809 Mary Cumpston Estimated Birth Year: abt 1809. Death - 1884 Q: Jul-Aug-Sep Age at Death: 75 District: Gateshead County: Durham, Tyne and Wear Volume: 10a see 1884 below
1846 Mary Ann Cumpston Birth 1846 Q: Oct-Nov-Dec District: Durham (1837-1938) County: Durham Volume: 24 Page: 80
1850 Medomsley P000692 Durham COMSTON Mary daughter of Andrew and Elizabeth Mackie IGI christening 19 May 1850
Deaths Dec 1846
Cumpston Mary Ann Durham 24 70 This cannot be the same Mary as she died before the document was written
Deaths Sep Q 1884
CUMPSTON Mary age 75 Gateshead 10a 537 I suggest this is the same Mary and am pursuing her death certificate.
Deaths Jun 1886
CUMPSTON Mary Ann 62 Durham 10a 1 [I suspect this is not the same Mary as there are other facts linking her to a full family, with a marriage and death.]
C 211/38/12 33 Victoria: Cumpston, Mary: of Dunston Lodge Asylum, Gateshead, Durh 1870 Mar
'PUBLIC RECORD OFFICE FILE REF L211/38/12/ 85642
10th March 1870
In the matter of
A supposed Lunatic
Left 14 March 70 
Johnston & Master
19 Southampton Building
Filed 20th April 1870 HWH
Durham to wit - the Inquisitors taken at the house of William Garbutt situate at or near Gateshead on Tyne in the County of Durham commonly called or known by the name of Dunstan Lodge this nineteenth [crossed out] thirteenth day of April one thousand eight hundred and seventy by xxxxx[unreadable]
Samuel Warren Esquire one of the Commissioners of our Lady the Queen by virtue of Her Majesty's General Commission under the Great Seal of Great Britain bearing date at Westminster the twenty sixth day of February one thousand eight hundred and fifty nine to Francis Barlow and Samuel Warren Esquires the Masters of Lunacy directed and under an Order of the Right Honourable Sir George Markham Giffard one of the Lord Justices of the Court of Appeal in Chancery made on the tenth day of March now last past and which Order is to this Inquisition annexed to enquire of the Lunacy of Mary Cumpston Spinster residing at Dunstan Lodge aforesaid He the said Samuel Warren having personally examined the said Mary Cumpston and taken evidence and xxx called for information to ascertain whether or not the said Mary Cumpston is of unsound mind finds that the said Mary Cumpston is a person of unsound mind so that she is not sufficient for the government of her manors messuages lands tenements goods and chattels And the said Samuel Warren certifies the same accordingly. The Testimony whereof the said Samuel Warren has to this Inquisition set his name and seal the day and year first above written.
Signed Samuel Warren.
Two £1 stamps
Overstamped with Chancery Court
Thursday the tenth day of March in the thirty third year of the reign of Her Majesty Queen Victoria And in the year one Thousand and seventy.
In the matter of Mary Cumpston, spinster a supposed lunatic.
Whereas the Commissioners in Lunacy have by their Report bearing date the fourteenth day of February One Thousand eight hundred and seventy Certified that in their opinion the property of the above named Mary Cumpston who is detained as a Lunatic at an Asylum called Dunstan Lodge situate at Gateshead in the County of Durham is not duly protected and that the Income thereof is not duly applied for her benefit and such Report had been filed with the Registrar in Lunacy Now upon reading the said Report an Affidavit of James Cambage filed in this matter and the Exhibit therein referred to and a further Affidavit of the said James Cambage filed in this matter of the due service of notice of the said Report on the said Mary Cumpston
[Down the left hand side of the paper is written] By virtue of Her Majestys General Commission and of this Order I have this day made the Inquiry thereby directed as appear by the Inquisition herewith annxxxx
Signed Samuel Warren 13th April 1870
On the first day of March Instant I do order that Francis Barlow and Samuel Warren Esquires the Masters in Lunacy or one of them do in pursuance of the general Commission under the Great Seal of Great Britain to them for that purpose directed enquire concerning the alleged lunacy of the said Mary Cumpston detained as a Lunatic as aforesaid And I do order that the said Inquiry be held at the place of abode of the said Mary Cumpston or as near as conveniently may be.
Stamped with Circular Stamp 'In Lunacy Register'
Signed by GM Giffard
[With initials LJ ? Lord Justice]
Please contact me if you can claim Mary.
Her story is very sad and I would like to pass to her family my copy of her Public Record Office file.
Mary Ann Cumpston born about 1825 wife of William coalminer born Chester? Mary was born Philadelphia, Durham, England Durham St Giles, Durham.
She is not mentioned on the 1841 or 1871 census.
She is recorded on the 1881 census with William,
at Stockley Durham. Aged 56 she is living with William her son aged 24, coal miner, Jno aged 21 also a coal miner, Mary Ann daughter aged 16 and Elizabeth Richardson daughter aged 14. There was also a younger son Geo. Cuthbert son aged 11, scholar.
33 Victoria : Cumpston, Mary: of Dunston Lodge Asylum, Gateshead, Durh
Covering dates 1870 Mar
Availability Open Document, Open Description, Normal Closure before FOI Act: 30 years
Held by The National Archives, Kew