Thank you for the replies.
I agree that the design is poor and it is a lovely bit of panelling on the interior.
Unfortunately the neighbour has had the same window remade and the client really likes them.
I wondered if anyone had any thoughts on the best jointing method for this area circled in the picture below.
It’s where the the two curved members meet and are then meeting the transom from above, with the central mullion joining from beneath.
There has been a failure on three windows...