Trying to retrieve an old o2 email account

[JBJ]

I registered a second Facebook account years ago using an o2 email address as the only registered contact. I have never used this o2 email. I now appear to be locked out of my desktop version of Facebook and the only way they seem to offer to unlock is by sending a verification code to this o2 email that I have no idea how to access. My o2 account is registered under my everyday email account. All very frustrating - if anyone has any ideas on how I can fix this I'd be very thankful! 

[MI5 · Accepted solution]

@JBJ 

O2 email closed down years ago.

You'll need to contact facebook to change your verification method.

[JBJ]

That seems very much easier said than done! Any advice on the best way to actually contact them?

[MI5]

@JBJ 

Not something I use much myself, but some help from FB here https://www.facebook.com/help/283100488694834/?helpref=hc_fnav