Thanks for the replies so far. No joy yet, however.

None of the Amadeus sites return results for either the old or new PNR. No success either on various LH, LX, BD and LO sites.

Requesting a passenger receipt from LH online brings the following message:

Der Original Passenger Receipt wurde bereits erstellt und kann nicht erneut abgerufen werden. / The original Passenger Receipt was already printed and can not be accessed again.

As reported in my OP, this ticket was issued in a hurry at the airport -- airport staff (LCY) and LH reservations were being very helpful and rerouting us to avoid connecting at FRA on a day when they'd already cancelled a stack of domestic German flights. This involved reissuing BD award flights with new LH ticket numbers and a new PNR.

Unfortunately, the new itinerary via ZRH didn't go smoothly. Our flight out of ZRH turned round for technical reasons a few minutes after take off, returned to ZRH and was then cancelled. We were rebooked onto a later flight. Our new BPs have the eticket numbers written on by hand.

That later flight was also delayed and we reached our destination over five hours late. Our bags arrived the following day.

LX is claiming "extraordinary circumstances", so a report to the BAZL seems in order. We don't actually need a receipt for this, just a printed itinerary that will show we had confirmed reservations. Submitting the old itinerary (showing a different routing on a different airline with different ticket numbers) would only confuse matters. Any thoughts?