Header image shrunk with new update

My header image has shrunk with the new update of the theme. Please see www.mummasminimes.com and you will see immediately what I mean.
I have had this problem before with the theme when I have updated a while back but it then fixed itself somehow in the next update.
I usually upload the image to the FTP and for the last couple of updates it has been okay and was the correct size until this latest update and I am back to what the original problem was back 3 or 4 updates ago.
I play with the header size and all the available options in the theme to no avail.
Any ideas to get it back to a normal size would be very much appreciated. Thanks!