Are you 100% certain its coming in under the cill, and not either through the glass beads or where the frame meets the cill? Also is this a load bearing bay? if so check on the corners outside where the bay pole goes through the cill as it may have been cut through to take the weight its possable the water is getting in this way well placed silicon will stop this from happening.Another common mistake is the cill has been fixed through to the timber frame before the frame was fixed in place so any wind driven rain will travel down the thread and sit on the timber.
If i was to get this call out first thing i would check is external seals as you have done then the base of the bay poles, then id remove bottom glass bead and pour pint of water in to window frame to make sure drainage is working.if drainage is working dry off frame and smear any screws in frame fixed to cill with silicone and finaly if you can see the back of plastic cill where the frame meets cill run a bead of silicone here aswell.
If none of these stop the problem then you may need to accuire about having the frame removed and refitted.
Thanks for that. I got their guys who fitted it out and they removed the glass and pushed the frame forward. They had screwed through the window sill into the wooden frame but there was a drill hole without a screw. Water was getting onto the top of the cill and draining through the hole into the inside of the window sill and out through a screw thread further along. Thay have bunged it up with a big load of silicon. Seemes to be ok at the moment, I will monitor it for a while just to be sure. This isnt a load bearing bay (no bay poles) just timber frame, chicken wire and render
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