getting damp patches just below the window on 1st floor

This is condensation, the wall is cold and the humidity within the property is reaching dew point on the cold wall. The best treatment is distilled white vinegar, don’t add water. If you’re serious about a permanent solution, apply a polyurethane backed plaster board to the wall as insulation.

You need to make sure you have plenty of ventilation in your room for air circulation also you could paint the area with dpc paint also a dehumidifier would do no harm good luck

To be absolutely honest without a proper inspection it could be a number of things bay windows are notorious for water ingress as well as condensation is the wall a solid or a cavity ? without knowing this it’s really educated guess work at best I advise getting a reputable damp proofing company to send a representative around to give you the correct course of action or you can just keep on trying different things until it works but best of luck with that !