Lucio Sponza's Italian Immigrants in Nineteenth-Century Britain: Realities and Images, Leicester University Press, 1988, is a good source of information on Italian plaster figure makers in nineteenth-century Britain.
Pellegrino Mazzotti was only one of many Italian immigrants in nineteenth-century Britain. By 1851 there were probably about 3,500 Italians in Britain, of whom about 45% lived in London. By 1881 there were about 6,800 Italian immigrants in Britain, children born outside Italy not included. The percentage living in London had risen slightly compared to 1851.
During the period from the 1820s up to the 1870s there were few changes in the occupational structure of the Italian community in Britain. It shows itinerant trades, like organ-grinders and other street performers, and makers and vendors of plaster statuettes on the one hand and some crafts, like the manufacturing of precision instruments, looking-glasses and picture frames on the other hand.
The Italian immigrants in Britain mainly came from mountain areas in Italy, where seasonal migration was an ancient custom. The plaster figure makers, also called image makers, plasterers, moulders, modellers or statuette makers, nearly all came from the province of Lucca in Tuscany. 'They never emigrated on their own, but moved in groups of five to seven people under the leadership of a master, who would be the most experienced in the trade (...). Often the other members of the team were young apprentices, aged from 12 to 15, (...). These small teams left the Tuscan hills towards the end of winter or in early spring. The master was normally the 'moulder' (...), one apprentice was the 'cleaner' (...), who polished the images as they came off the mould, another apprentice was the 'painter' (...), who coloured them.' (Sponza 1988, p. 76) The other members of the group sold the images.
'(...) only about 10 per cent of the Italians engaged in itenerant occupations were thought to become permanent residents in Britain (...). (Sponza 1988, p. 104)
Another source on Italian plaster figure makers is Paola A. Sensi-Isolani (1990) 'Italian Image Makers' in: Sensi-Isolani, Paola A. and Tamburri, Anthony Julian, eds. Italian Americans celebrate life: The arts and popular culture. Selected Essays from the 22nd annual conference of the American Italian Historical Association. [S.I.]: American Italian Historical Association. http://www.fazzi.nl/figuristi-di-gesso/fazzi/italian_image_makers.htm : accessed 13 January 2013.
Further information on British plaster figure makers, including some Italians, can be found in the introduction on plaster figure makers by Jacob Simon on the National Portrait Gallery website.