posting an update to say that Autoglass phoned yesterday and confirmed that the insurer agreed to replace with a Ford windscreen and I am now booked in for the windscreen replacement.  Thanks again for all the advice here as otherwise my mums just over a year old car would have had a non genuine glass fitted and since she is keeping the car hopefully for next 10 years I think well worth insisting on genuine manufacturer replacement.

Having read the posts again above I too have opened the car door forgetting to put the engine off because it is so quiet so soundscreen glass definitely makes a difference and is not something I had heard of before.  Looked at the current windscreen and took a picture so I have all the details of the current screen to compare when the new one gets fitted.

I will post an update once the job has been done

so far so good with the windscreen replacement.  Checked the details on the glass against a picture I took of the original and yes an exact genuine replacement.  We have had very heavy rain and no obvious leak issues and yet to try the heated front screen.  

Once again thanks for all the input on this thread - it has been invaluable 🙂