My last bill payment went out on 1st for £35, Orange let me out of my contract on 3rd and gave me my PAC code same day, it transferred over on 5th, So on 5th I was no longer with Orange,

I received my final bill yesterday for £20, Now I was under the impression you pay in advanced is that not correct and as I left soon after the last payment went out should they not actually be owing me money?

Normally the way it works is:-

Example Billing date 15th

Last Bill covers 15th -> 14th taken 14 days later by DD.

Final Bill = credit from 6th - 14th, however 30 days notice from when PAC was requested so debit from 6th - 3rd.

This should fully be explained when leaving networks, as even though the PAC process is now very quick you still pay for the 30 days notice.