The first time a reference is made to "the little gray cells," Hetty says "red cells" by mistake.
Not convinced that the first quarter of this was actually filmed in Pisa; the single shot of the Tower appears to be a photograph or painting rather than a live shot. The opening scene does not have any of the characters appearing in front of the Tower and could easily have been used from archive footage of another production. The bus they are travelling on throughout the Tuscan countryside is driving on the left side (common in UK) and not on the right side (as in Italy). But mostly, this is only the third production of the first series, it is doubtful the budget would allow for a cast to be flown over and lodged in Pisa for a brief five minute scene in an hour long production.