Looks to be welded in one Gaz, top right I think you can just make the weld out viewed inside.
The fitter is clutching at straws if if thinks he'll replace the side screens, will look naff as well
This install needs to go though LABC to be signed off, depending what inspector you get, if it's below dpc it could fail
Should have trickle vents as well
Total mess!!
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local