If my memory serves me I believe there was a major IM failure at LHR. The Capt was told not to divert to LTN but choose to do so, this was in some part due to presence of a towbar, the intention to wait and then position to LHR. After landing it transpired there was no towbar therefore a/c had to be disembarked without the baggage.
Crew ran out of time anyway and the towbar took some time the aircraft only getting back to LHR about 10 minutes before the night ban.