NHS Property Services published in May their investigation report about the Weybridge Hospital fire in July 2017.
The report says the most probable cause of ignition was an air-conditioning unit on the second floor of the building and the resulting fire was made worse due to a combination of other issues associated with the electrical installation and fire protection in the building.
The air-conditioning unit was in a ‘server room’ housing IT equipment on the top floor relatively near the front of the building. This is why it was this part of the roof which was first seen ablaze as in the above picture.