Help investigate old walled garden

Hi all,

My wife and I recently bought a property in South West England within a country estate. Our garden consists of one third of an old walled garden used by the country estate, and we are desperate to try and identify what structures used to be in the garden based on the images shared.

I have attached a photo of the walled garden, with the section of our garden highlighted in a red square (suspected to be from early 1920's). I have also attached an image of our current garden, to provide context!

I am intrigued by the large white looking structure in this walled garden, would anyone have any ideas?

For further context, at this time the estate had been turned into an impromptu hospital with extra buildings added.