Leak water marks inside below and to the left of Bow window

If the bow window has been in place for around 20 years without any previous issues, I'd be looking for something that has changed rather than assuming the window itself has suddenly failed. Some of the more common causes are: Failed or cracked external sealant around the frame. Deteriorated pointing or hairline cracks in the brickwork above the window. Defective roof or bay flashing (if the bow has its own roof). Blocked or defective window drainage (weep holes). Failed cavity trays or damp-proof details above the opening. Overflowing gutters or leaking downpipes allowing water to track into the wall. One thing I'd avoid is simply applying more silicone until the source has been identified. In many cases this only masks the problem and can sometimes make diagnosis more difficult later. My advice would be to have someone carry out a systematic water ingress inspection, starting externally and working methodically from the highest point down. An experienced surveyor or builder who specialises in diagnosing water ingress is often the best place to start, as they can identify the source before recommending the appropriate trade, whether that's a roofer, window installer or bricklayer. If you could upload a couple of photographs of the inside staining together with a few clear external photos of the bow window, including the roof (if there is one), it would be much easier for those of us on here to point you in the right direction. Hopefully it's something relatively straightforward rather than anything major.