When Burgo calls at the Palliser's London house, the exterior of the house shows number 25. Later, in Episode 7, Phineas Finn asks the cab driver for the fare to 24 Park Lane.
(at around 38 mins) An exterior shot of the Palliser house clearly shows the house number is 25 Park Lane. A little later, Phineas Finn asks a cab drive for the fare to "24 Park Lane."
As two Victorian gentlemen enter a conservatory to the right, the sound boom is seen being swung slowly away to leave to the left at about the 6 minute and 35 second mark.