The first step is to write her a formal letter or email requesting return of the hat. When you said you left it do you mean you left it by mistake and she then asked to borrow it or do you mean she asked to borrow it and you lent it to her?? either way in the letter set out the facts accuratley but formally.
Set out the facts like this
On X date I lent you (insert formal description of hat - a Philip Treacy hat that is a pink pill box hat with a long green feather).
(if there was any discussion about length of loan include is as in . 'You told me you would like to borrow it for friend's wedding on X date and I expected to have the hat returned relatively promptly after that date).
I have asked you to return the hat X time as follows
by email on date
by telephone on date
in person at Joes Cafe on date
You have failed to return the hat. This is a designer hat of some considerable value - several hundred pounds - and also of personal sentimental value to me.
Regrettably, I now have no option but to formally ask you in this (email or letter) to return the hat immediately and by X date at the latest. If you fail to return the hat to me by that date, I fear I will be forced to take legal steps to recover the hat or its value but obviously as you are a friend that is really a last resort.
I am only writing to you in this way because I have asked so many times and you are refusing to return it. Please let me know when you will be returning the hat on a date and time before X date.
Sorry if this sounds very formal but like I said, I feel I don't have any alternative.
THEN
if you get no where, look at small claims if you can be bothered - I don't mean that in a perjorative way just that it will be a lot of "work" and effort and it depends whether the hat means that much to you.