Hi, I am in the same situation as yourself, sent old passport from within the EU by DHL, on 5th May signed for in the Durham office on the 6th May still not received by Durham and e-mail asking for old passport to be sent, i called the help line today and was informed no help can be provided until i get mail stating passport received.
Crazy situation, I’m lost for words at this stage, you cannot use fast track service when abroad so its only luck that will get us through this nightmare, i will try calling the help line again next Monday.