@blackhall you must write they won’t accept calls. If your going to do it send multiple letters
Mine included
My name and address DOB
My reference PEX
my lost passport reference L
Why I was withdrawing
That I understand I would lose my fee.
Then signed and date which must be in ink
I confirmed where my application was held (cos they can be moved about) and sent them guaranteed 1pm next day signed for. I also emailed copies of the letters and proof of delivery.
I also called and asked them to log I was withdrawing as I had secured a fast track and gave them my booking ref to add to my notes.
Hope this helps.