Tbh... No streams of water on inside of glass, and no hot tub (unfortunately). Can't see any drainage holes either. Not too sure exactly where to look. Taken some more pics. Thanks again for your help.
Have lived in our house for around 3 years now, and this has got progressively worse every autumn/winter. Have had a couple people look at it, and they all say something different. Have topped up the sealant to the uPVC windows on the outside. House is semi detached, and around 100 years old. We...