Hi
Shenley mental hospital was situated on the outskirts of Shenley village which lies between London Colney and Borehamwood.
There was a church located on the edge of the village called St Botolph's which was subsequently replaced in 1839 by a new parish church, St Martins located centrally in the village.
The churchyard for both churches is opposite the original Shenley hospital site and can be found next to the old original church - this area is known as Shenleybury.
At the entrance of the churchyard is a board of names, dates and positions of graves. I have found relatives of mine who died in the 1940's & 50's who are buried there.
If you have no luck, I don't mind having a look at the chruchyard site when I am next in the area.
Good luck.