As far as I can gather the 876 to DME was cancelled and routed LHR-URA-LHR on aircraft #1 which would account for some of the pax.
The LHR-OTP is a nightstopping service so operated as normal to OTP. The crew due to return to LHR then operated OTP-URA-DME-URA on aircraft #2. This would shift some pax up to DME to take the evening flight back to LHR on aircraft #3.
A third crew then positioned LHR-DME-URA to operate aircraft #2 back to LHR bringing with them the crew who'd done the OTP-URA-DME-URA leg and they'll be back about 5 am tomorrow!
All in all a good job done by all getting a lot of people out of an offline station at short notice.