WILKS, Metropolitan District Asylum, Leavesden, 1881-1915
Information on the inmates is readily available from the census returns and I note that a Sarah Wilks, a married 68 year old lunatic, born in London, was a patient there at the time of the 1881 census. If she is the one you are interested in and did not die until 1915 she would have been over 100 and should be listed in the 1891 and 1901 census returns which you should have no great difficulty in checking.
As to any hospital records they would probably have been transferred to HALS in Hertford (as happened with the records for Hill End Hospital, St Albans) or the Metropolitan Archives in London and an enquiry at either of these places would probably tell you what happened to them. However it may well be that patient records (which would have been nothing like modern hospital notes) have not been kept or are held under a 100 year embargo. If you are lucky there may be surviving admissions and burial books.
If you can add to the information given above tell me.