Wow!!!
I've slept a few times since then John.
Not sure if it was the Granada or an earlier Opel but it was inside one of the air outlets on the dash. It had worked it's way through the air system and got stuck behind the grill. It dried out with the airflow and started whistling.
I was about to emulate a song but decided not to give you a laugh.

I have found the area of concern:

https://youtu.be/SFCWzNaUr3Y

So it seems I’ve damaged something, left something out when putting things back together after replacing the door handle.

Odd, cos I had no ‘bonus bits’ left over from this job. If it’s dry tomorrow I’ll have a proper look.

Did you find out what was causing the noise. I think I have the same issue, previous owner also replaced 3 of the handles.

Yes mate. It was the window seal/trim not seated correctly. Packed it out a little with some neoprene type stuff as replacing the offending article seemed like quite a task.

Didn’t like the ‘bodge’ in all honesty, but soon forgot about it as the noise stopped.