How to find passport application number?

I had a fast track appointment at the London office on 12th April, then had a text saying I should receive my passport 'in a few days' on April 19th. There's been no sign of it however. I'm trying to do the passport tracking, but it needs an application number, which it helpfully tells me is on the front of the application form I no longer have. Is there any way of finding out your application number without spending hours on hold to the helpline?