Phoned up on Monday and they advised me to report it lost, did so, and now it’s been marked as “received” (i.e they can move forward with the application without receiving the old passport).
No Royal Mail search but I did provide them with the tracking number. They requested April 1st as the proposed completion date but heard nothing from Hemel Hempstead itself.
Contacted them on Friday and yesterday. Both similar responses, they wrote messages to the HH Office to chase it up and said wait 7 days for an update. Not filled with confidence.
If your delivery was signed for, then they most likely do have it; there seems to be a real inconsistency, however, in terms of how quickly passports are renewed.
Similar situation with me- application submitted on 27th Feb, arrived at office on 1st March but application tracker still says documents not received.