There was a thud and fairly substantial jolt while in cruise and the Captain announced that we were diverting to Boston to investigate the cause, which they thought may have been something in the hold. Once on the ground they checked the hold and found nothing untoward, then after further checks found no obvious cause so (quite correctly) abandoned the flight on safety grounds.
Pax were all transferred to hotels and the next info we had was by SMS message the next day (24/6) saying that they were examining flight data and awaiting approval from Airbus and Engineering prior to departure. This was followed four hours later by another SMS message saying that "the aircraft cannot be made serviceable anytime soon" and that another aircraft was being despatched from the UK to pick up the stranded pax.
So, in short, there was some issue with the aircraft but what that was wasn't communicated to the pax. Registration of the A340-600 concerned was G-VWEB and looking at the Virgin aircraft database it next flew BOS-LHR on 28/6 as VS122 which is not a normal scheduled service.