I was actually at work that day and id like to add that nobody from the airport had actually asked servisair or any other handling agents if they had a towbar until the a/c was on the ground.
Also the passengers were also kept on board for that amount of time as the c/ crew were still in hours and when it became apparent that they weren't leaving in any hurry BA made the decision to offload the pax and transport them back to LHR.
One last thing the towbar problem was already being sorted once the a/c arrived on stand at LTN there were two towbars sourced and dispatched to LTN one from LHR and another from STN
So it was not Servisair's fault but more an airport problem for accepting the a/c without speaking to anyone.