Once you’ve located the draught, you’ll be able to do something about it.  This something usually comes in the form of a replacement compression seal.  These rubber strips are designed to grip the side of the window, and will compress under pressure.  You’ll need to cut yours to size and glue it to the window frame – following which, it’ll form a tight seal whenever you close your window. Such seals aren’t appropriate for sliding sash windows, as they’re designed to be squashed rather than rubbed against.  Instead, you’ll want to use brush-strips.  These will sit alongside the interior of your window, and allow them to slide against them bristles, which will still help exclude the majority of the draught while ensuring the window can still be opened.