uPVC window - gap in material beneath sill

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Been cleaning/restoring our uPVC window frames. Noticed that the sill was loose so removed it to give it a proper clean. Underneath is a gap in the capping whcih doesn’t look right to me. Imagined that this should run right underneath. The seams of the window frames/seals were sealed with silicone sealant so I’m guessing the previous owners/builders sealed them up to prevent water getting in (previous owners had cowboy builders renovate the property).

What is the best way to fix this?

Appreciate any help. Thanks
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If the frame is securely fixed, you could fill the underside with foam, them add a wooden sill butting up to the frame with horns at each end, ensuring all is sealed to prevent ingress of water.

Blup
 
I can't quite believe what I am seeing in your photo, none of my windows look anything like that and I am cocluding that the window installer was an amateur (there are plenty of them about). That "gap" should not exist at all and although I don't claim to be an expert, I would say that the wall cavity should properly bridged with a robust material and then sealed with a suitable weatherproof silicone. THis may involve a bit of breaking on the inside (sorry)
 
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To me it looks like the window was measured wrong to start wig i.e didn't include the cill within the height size, so the cill was left off and they cobbled a fake cill up with that bit of flat board/trim stood against the wall to cover what the new cill would have and what the previous cill was covering, my guess it was a crittal frame in a timber subframe
 
Exactly my thinking, was just trying to check something upvc was removed before saying lol.
 
I am understanding a bit more now, if the cill were properly fixed, it would not be loose and you would not have been able to remove it. Plastic cills/capping boards are available in various sizes and are usually fitted before actually fitting the window frame. The frame then tightly fits down onto the cill provided the fitter has got all of his measurements right to the millimetre and a thin line of silicone is added with perhaps a thin piece of plastic board to keep water out. Obviously, none of this was done correctly and the bit of cill which you pulled off was added as an afterthought when they realised that they had an asthetic problem. "That will do, if it goes wrong we'll be long gone." So what to do now? Well my Mrs would say tear it out, I want it done properly, and she would be right but you may not want to run to that expense. In which case I would do as the man says and fill it with foam as well as you can then re-attach the fake cill using a suitable gap sealing adhesive. This will take a bit of DIY skill so if you are not confident, there may be a good handyman available locally like me, where are you?, I dont mind doing up to 20 miles.
 
Thanks for all the tips guys. After posting i brushed out all the debris and you can see the original cill with some extra bits stuck on, then covered up by the uPVC piece I removed. Thanks for offer @Eccles , I’ll be all good to fill with foam and seal.

Uncovering surprise after surprise with this house, should see corners these builders cut when the previous owners had the extension done!

Thanks again all
 

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