As this entry from the National Archives shows, the records of Burghill Hospital are held by Herefordshire Archives and Records.

https://discovery.nationalarchives.gov.uk/details/c/F150837

Unfortunately, it would doubtless take a personal visit to get at them and you don’t seem to be in a position to pop in for the morning!

They do offer a research service but at about £18 per half-hour plus the cost of any copies/imaging you might run up a sizable bill and end up with nothing to show for it. Mary Ann might a good bet since you have her date of admission.