Leaking roof

[Winnie1965]

Wondered if anyone could help had a brand new roof gutters and drainpipes fitted on house in April then 2 weeks ago during the torrential rain my ground floor bay window began to leak ( never had issue before) anybody have any ideas what the cause could be. Bay window is completely sealed so no issues with that.Roofer is at a loss as to what it can be he doesn't think it has anything to do with the new roof but I can't see how it can be anything else.

Maybe take some photos of the external areas and internal and indicate where the water is coming in and then perhaps @PigletJohn could help?

Is there a leaking gutter joint over the window allowing water to run down the exterior wall underneath it. If so there could be a way the water is getting in and running down behind any plaster and entering above the ground floor bay. We had a similar problem. With ours the leaking joint was above the left side of the bedroom bay but because of a slight slope it was running down the right side of the bedroom bay then down the front of the house and coming in through some hair line cracks above the bell casting over the ground floor bay.

If there is a joint anywhere near the Bays get the guy to mastic it. Good luck.