Who do I need ?

I have a mid terrace property and I have just noticed the internal party wall is damp. It had very old heavy wallpaper on it that I noticed was separated and when I investigated it came off. The wall is old lathe and plaster and looks a tad sweaty. I’m unsure if this is condensation or something more. The property it’s adjoined with has been vacant for at least 5 years. What tradesperson do I need to investigate the cause of this?