how to get rid of black marks appearing on wall paper in bay window and corners of outside walls

Hello The blacks marks are actually a mould generally caused by poor air circulation the solution is to wash the areas with a mould inhibitor to kill any spores and try to create an air flow either through a vent or leaving window slightly ajar Hope this helps Regards Lionel Bexon

It could be more serious than surface mould .... I am presuming this house is Victorian? We see it all the time in houses with solid walls and no damp proof coarse You may need some damp treatment, the best solution is normally to inject a resin damp coarse

Condensation issues...Poor air quality, needs ventilation and all mould cleaned up.

I agree with cannon and tradesman. Black mould only forms with clean water so it can't be ingress water coming through the brick work. Ventilation is needed put an air brick on the bay widow wall and or open trickle vents on windows if you have them. Regards Chris