Does anyone have code to transfer double number to array of characters for writing to Flash memmory
Make your own code.
If you want to save as ASCII, then use sprintf() to print to a buffer that you may write to the flash.
If you want to save in binary form, then just take the address of the double variable and copy to a buffer.
The flash does not care what you write to it. All you have to do is prepare a buffer of data and then write it to the flash.
Indeed, the Flash neither knows nor cares anything about the meaning of the data - all it sees are bytes to be written to addresses.
Whether those bytes represent executable code, ASCII text, numbers, of anything else is entirely irrelevant to the Flash.