I’ve had a look myself and it is indeed caused as Pucci says by the initial delay back into Heathrow, it then did indeed pick up a delay from
ATC due to ‘weather at destination’. As Karfa has shown this was due to CBs in the vicinity of LIS. It appears some bags needed to be removed from the cabin prior to departure too on the flight in question, but this was the most minimal of the reasons (in terms of minutes) for the delay. The major cause of the delays is the delay out of LHR to LIS owing to this slot delay. Other wise you delay would have been nothing more than 30 mins at most, if that.