Brock (1982) summarises the taxonomy and biology of British Ophion, including an analysis of the status of different forms of O. obscuratus. Other useful European references for identification include Oosterbroek (1978) and Izquierdo (1983).
Ophion obscuratus is just one of over 2,300 British species of Ichneumonidae. A checklist of the British and Irish species is available:
Checklist of British and Irish Ichneumonidae (Hymenoptera) PDF (1.1 MB)