Hello everyone , thanks for all the suggestions. I do have an update. I did make arrangements to go back to dealers since I am halfway thro a 2 week grace period when I can return the car for full refund. So it’s been difficult with current lockdown restrictions to do social motoring to diagnose the problem , especially as I need to get to a motorway to achieve the required speed to bring on the noise. So last night , before my trip to the dealers today , I had a good look round the bonnet and mirror areas. What I found , standing at the front of the car and looking towards the drivers side mirror was that there was a small gap , perhaps 1 or 2 mm, between the 2 plastic trim covers below the mirror. I think on my Civic this was a substantial piece of metal but on this 2019 Mk3 Jazz it is 2 pieces of plastic that fit together. This same gap was apparent on the passenger side mirror. I pressed both half’s together and the gap closed up , then did same on drivers side. Don’t know how these covers are supposed to be secured , whether they are secured with screws , some latch mechanism or are just a tight fit.
Anyway this is the only thing I’ve done to try and rectify the noise and on my trip to the dealers there was no whistling noise even at 70 mph. So I think I’m in the right area. I was disappointed at the dealers that neither of the 2 people I had discussed the problem with were available ( didn’t make me feel like a valued customer) and I opted not to have anyone else mess with the car. I’ll continue to monitor it, I have not yet swung the mirrors in and out to see if this makes the gap come back. Will just get confidence that this is the cause initially. Perhaps any of you Mk3 Jazz owners could take a look at their door mirrors and see if any gap is evident between the 2 plastic “shells” under the mirror.