Is my tenant sub-letting?

Other

I am beginning to suspect that my tenant may be subletting my flat. She works as a foreign diplomat, and is often away from home, so it is not easy to tell where she is living at any one time. She used to share my 2 bed flat (one adult, one child bedrooms) with her husband, but they split up and she took out her susequent ASTs as a single person.  She is often hard to contact but she explains any delay is responding to my emails as her work commitments. There has been a ongoing problem with water leaks from the flat above for a couple of years now, but recent visits by my contractor to redecorate have been let in by people I don't know and a recent email from her about the latest leak came via a man I don't know either - she had asked him to describe the waterstains to her and take photos. Perhaps she did not realise this email was still at the bottom of her message to me. The message from him even gave his email address. Of course, this person could be a friend or relative staying as a guest in the flat. It could even be a new live-in boyfriend or commercial flat-sitter she uses when she is away on work trips.But I cauld not tolerate her renting out my flat illegally to some stranger. Do members have any advice about finding out the situation without making any false accusations? And if it is sub-letting, how would they proceed?