During my identification and learning process (never ends!) I took photos of the bits of wasp specimens which were alluded to in the key(s), mostly first and last below, to help me understand what was meant:
Yeo P.F. & Corbett S.A. 1983 Solitary Wasps. Naturalist's Handbooks 3 CUP
Richards O.W. 1980 Scolioidea, Vespoidea and Sphecoidea RES.
and worth every penny for the clear illustrations and descriptions:
Lomholdt O. 1984 The Sphecidae (Hymenoptera) of Fennoscandia and Denmark. Brill (publisher, not a comment on how good I think it is!)
Discovered as new to Britain in 2017 by David Notton and Hannah Norman. The discovery was made from a specimen collected in a garden in Lewisham in May 2017. Subsequently a small number of specimens from southern England have been identified, dating back to 1802. Full details are given in: Notton, D.G. and Norman, H., 2017.