Fixing gap in windows

[SquashBandicoot]

There is a big draft coming through the bottom of our double glazed windows where the window meets the sill. I can see a crack where it has separated. Can I DIY fix this?

It's a bay window, 1910s house, windows at least 20 years old.

Although it is creaking in the current winds so maybe I should be worried.

I would just run some mastic sealant along that join, but I am no expert

I would prefer to fill the unseen gap between window frame and wall with expanding foam. It is very messy in use and extremely sticky. I had good results by covering the wall and frame with masking tape and clingfilm, and injecting through holes drilled at about 1ft intervals. As it expands it will push along the gap until you see it coming out of the next hole or gap. It is best to do small sections at a time, letting each finish expanding before you do the next one. If you put in a lot, and it has nowhere to expand, it can push the surfaces apart. You need less than you think. You can spread the work by doing top left, bottom right, centre top, centre middle and so on to let each section finish expanding while you do another. Once it has hardened you can cut off the excess with a sharp knife and smooth it with sandpaper, and finish with paint or white silicone. Externally it must be painted or sealed because it breaks down in sunlight.

Edit
And working from the outside where you may be able to see the gap better.

Having the same problems inside and out here.. outside looks like a much bigger job that I don't think I can DIY!

I would use frame sealant on that, get exterior grade, has a lot more durability than eg caulk. I would probably try to flatten the plastic nozzle that the tube will come with, to try and fill as deep as possible.