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The British list follows the taxonomy of Phil Withers (1989 and others), which is quite different from the opinions of most other specialists. In Europe, it has been the consensus since the early 1970s to consider the Pericoma of old to consist of multiple genera, based mainly on differences in larval and antennal characters. Useable keys to these genera can be found in Vaillant (1971), Wagner (1997) and Krek (1999).
The main problem with Pericoma is that it is not yet satisfactorily defined by any author. Traditional characters used to define it in Europe and the Palearctic Region (flagellomeres not differentiated into main bodies and necks, sensory filaments of flagellomeres digitiform or reduced, male genitalia symmetrical) are all plesiomorphies that occur in several independent lineages of Psychodidae elsewhere in the world. The most obvious example of this is the primarily African genus Clytocerus, which also occurs in Europe, Northern Asia and North America - it shares the defining characters of adult Pericoma, but is upon closer examination quite distinct in the structures of the male genitalia.
Pneumia is best defined in Vaillant (1976), who calls it Satchelliella due to misunderstandings about zoological nomenclature. The genus was most recently redefined and delimited by Omelková & Ježek (2012). I use the name mostly since the extent of it is relatively unambiguously understood - other specialists understand what I mean when I use the name better than if I had lumped it with Pericoma s.str.
Hope this is of any help.

References:
Krek, S. 1999. Psychodidae (Diptera Insecta) Balkanskog Poluotoka, Studentska Stamparija Univerziteta Sarajevo, Sarajevo.
Omelková, M. & Ježek, J. 2012. Two new species of Pneumia Enderlein (Diptera, Psychodidae, Psychodinae) from the Palearctic region. Zootaxa, 3180, 1-18.
Vaillant, F. 1971. Psychodidae - Psychodinae. Pp. 1-48 in: Lindner, E. (ed.) Die Fliegen der Palaearktischen Region. Lief. 287. E. Schweizerbart'scheVerlagsbuchhandlung, Stuttgart.
Vaillant, F. 1976. Psychodidae - Psychodinae. Pp. 183-206 in: Lindner, E. (ed.) Die Fliegen der Palaearktischen Region. Lief. 313. E. Schweizerbart'scheVerlagsbuchhandlung, Stuttgart.
Wagner, R. 1997. Family Psychodidae. Pp. 205-226 in: Papp, L. & Darvas, B. (eds) Contributions towards a Manual of Palearctic Diptera. Volume 2. Budapest: Science Herald.
Withers, P. 1989. Moth Flies Diptera: Psychodidae. Dipterists Digest, 4, 1-83.