Both Mazzini and Carlyle are almost always depicted with a beard but finally we found Carlyle displaying his chin and that image looks to be the one used by the sculptor, it is so close. So we are convinced: Mazzini on the left with his friend, Carlyle on the right.
Site: Cheyne Walk - friendship (2 memorials)
SW3, Cheyne Walk, 15
Wikipedia has a page about this house which name-drops many celebrities but none of the 4 depicted in these sculptures. The sundial between these two double portrait plaques is headed "Lead kindly light", words from a Cardinal Newman hymn. Built c.1718 this house is Listed but the entry does not mention any of the items on the front elevation.
Then we found a detailed 2004 archaeological survey of the house which tells us that the portrait heads do not appear in a photograph of the house in the Survey of London (1909, plate 70) so they were erected after 1909, and also suggests that Lord Courtney erected them. That's the Liberal politician Lord Leonard Courtney of Penwith (1832-1918). He was in the house from about 1883 until his death and then his widow stayed until her death in 1929.
We then found the book Life of Lord Courtney by G. P. Gooch which says, in Courtney's widow's words, talking about the changes her husband made to this house: "Then came the sundial — an old one fixed on the front of the house. The motto on it was his choice — ' Lead, kindly light.' But his biggest venture was the two pairs of sculptured heads — Sir Thomas More and Erasmus on one panel, Carlyle and Mazzini on the other."
So that's the heads identified. Only two questions remain: who sculpted them? and why these 4 men rather than anyone else? We cannot discover the name of the sculptor but the other question turns up an interesting result.
From Byronico we learn that Mazzini lived in a number of different Chelsea addresses at different times (although Cheyne Walk itself does not figure). He was good friends with Thomas Carlyle and would sometimes stay overnight at 24 Cheyne Row. Thomas More lived in Chelsea as is well-documented on about 6 other memorials to him in the area, and Erasmus was a good friend who stayed with him when in London.
So each panel shows the patriarch of a Chelsea household alongside a very good, foreign, friend who was often"put up" in that household. This pair of paired portraits commemorates, not just 4 illustrious men of history, but also long-lasting male friendship across national borders.