I think it was closer to 3 months to prepare for the repair a five weeks estimated repair time. Perhaps a good deal of the three months was putting into action a well-researched operation, but Boeing did apparently build a whole new rear fuselage barrel to cut the repair patch from. I presume this would have taken quite a while, and in the mean time they had to plan/hire/build the shelter and heating/ventilation equipment. There is quite a lot of "stuff" there; sourcing, making and/or hiring all of that would take a while, especially as composites need controlled stable environments, and winter at LHR is hardly stable!