More than 50 firefighters and eight fire engines were called out to the blaze at GHQ Coleshill, at about 04:30 am this morning.
Crews were forced to work in “arduous conditions” but managed to stop the fire spreading to the rest of the building.
Group manager Kerry Blair said: “This severe fire has devastated a family-run business.”
He praised the “tireless” work of fire crews from Oxfordshire and Wiltshire and said they were working with police, paramedics, Southern Electric and the Environment Agency.
“Fire crews will be on scene all day damping down, and representatives from National Trust are already in attendance to manage the welfare of their tenants,” Mr Blair added.
‘It is well known that the Admin offices which adjoin the building which burnt down are in a terrible state and largely full of junk (Left). It’s a credit to the fire service that they have managed to save these buildings which the National Trust hope to one day restore.’ says Tom Sykes, Founder and Webmaster of the British Resistance Archive.
The image above shows the main building that has been destroyed. The upstairs rooms were used by the Auxiliers to sleep in when they were training onsite. See a video of the inside the admin offices here.