quick update - I finally managed to get a sample of three smooth snakes examined by an expert vet yesterday which all appeared gravid to me, and he was able to confirm that they had reasonably advanced young inside already. So this would seem to show that smooth snakes are not only capable of carrying their young over winter but also that this is a reasonably common thing, rather than a one-off due to an individual snake being in poor condition or an unusually cool summer in the previous year. More data needed, but it's a start. Next question, are these embryos the result of mating last spring, or, as has been recorded recently, autumnal matings...?