I have 3 pairs of Fly boots and love them all. FYI - the sizes come up big, but there is also inconsistency; I am usually a 4.5 in most shoes, sometimes a 5. But when I got the Fly Yule boots I ended up settling on a size 3 which was a bit tight at first but stretched - the 4 was massive. My Fly Midas boots, however, are a 4 and that is a perfect fit for me. So don't rely on your normal size is my advice.
Also, the calf size is not a problem - my calves are quite slim but I have friends with much heftier calves who have bought Fly boots with no problem.
Anyway at £40 you'd be a loon not to get them. The calves will stretch.