Hello,
Thank you for showing that hat. While I am not quite as familiar with London milliners as I am with French and American ones, I have seen a few of Clark's hats over the years. I don't know the range of years she was making hats. Judging by the style, it looks early 1970s to me. Also, the straw she used (which looks to be a very fine & expensive buntal or bali-buntal straw), and the other materials, construction methods, inside, etc., all point to the pre 1980's time period.
Getting out creases is possible, but not always successful. Where is the crease? If it is the straw brim or crown, try steaming...but if the hat has been sized (and it looks to have been), the sizing can make the hat sticky or buckle, or the hat can re stiffen in the wrong shape, etc. Handle with care and try to steam out the wrinkles or creases and then stuff the hat or place something over the area so the crease does not come back, and let it cool. Use clean fingers to coax it back...but again be careful of the hot steam and take care not to handle the hat too much. It may work, or it may come out worse, depending on your expertise in this area.
If it is in the wire in the brim....this is difficult to ever get out completely, but you can try to re-shape the wire by hand.
B