iv been a windsecrren fitter for ten years, the reason lexus insurance direct to national is cus they are the approved repairer for that insurer. 9 times out of 10 if you take it to the dealer to have done,national will do it anyway. although some of the larger dealers will have an inhouse fitter.
If national do the job,insist on a dealer part only trims and clips,this is your right!
when the fitter comes he should do a pre check and physically check he has the correct screen, check for rain and light sensors and test, tint of glass.
test once replacment has finshed.
have a good look around work area before signing!!! look for scratches to paintwork(tops of wings,bonnet edge near screen,inner an outer A posts,roof and lining).
hope this helps.