Leak on my kitchen window..

Firstly there should be a cavity tray in the cavity to direct water out through a weep hole the water could be coming in anywhere if there is no cavity tray which sounds like there isn't if you can see no obvious entry point

I believe you could possible have bridging of the cavity with mortar rubble. If water is just one side the tray may be leaking one way. Possibly no try at all, you should have weep holes above the window which drain water out if not you may need to install a tray to stop water leaking in. In most cases you can only determine what the problem is once you get in to take a look.

You would need to do a visit. Reason being it could be cavity brickwork it could be solid brickwork it could be blockwork we don't know secondly is it an upstairs window with guttering above or a downstairs window so therefore you would need to have a visual inspection to determine the problem

Depends on how old your property is newer property’s have damp trays and that should prevent water from coming in and go out threw weep vents Older property can have lots of issues depending upon age ,just a case of investigation