I know that two APTC members of us have had inspections - Juliet (Erith) last year, and now myself, June 2016.
If you are Ofsted registered and have not had one, this was how mine went:
The inspection was carried out on behalf of Ofsted by an agency called Tribal (they have a website).
I received a phone call on a Thursday evening (from Private number: of whom I am always suspicious) to arrange a visit the following week. I provisionally agreed, took the details and looked up the website shortly after.
The inspection lasted about 45 minutes (an early morning - there were no students present) and was mainly about checking compliance with the Ofsted requirements: up-to-date DBS certificate, First Aid certificate and public liability insurance. I was quizzed about my complaints procedure and safe-guarding.
The follow-up report arrived by registered post very promptly. It was brief and confirmed that I was meeting the essential conditions for registration on the Voluntary Register.
Tribal were keen to have feedback on the conduct of their inspector - I couldn't fault mine for her method or manner.