if u take the plastic "dashboard"/facia
off u can see where its getting in

I've done it already! Indeed it comes from the flap (the rubber is not too old, it is still flexible),
the windscreen rubber or the tin frame, and curiously also from the axles of the windscreen wiper. I say curiously because these axles have been substituted recently...

Are the rubber rings under the wiper spindle nuts ok?

If you try to stop any leaks from the vent flap by tightening the close/open knob, all you do because it's attached centrally is pull the middle in, the end will then pull away from the body causing a bigger leak. All you need to do is use some self adhesive draught excluded run around the vent hole and boot over tighten the flap

Yeah, those rubbers are OK. They are new. I guess it's the axle itself.
As for the flap I will probably have to substitute the rubber again and check the tin under the top of the flap.