Many thanks for the reply.
I agree: it works if one doesn't use the mobile version. Although my experience is that, having successfully scheduled something with the PC web page, the mobile version still fails to schedule a (different) show. And you are right - it didn't see Backchat or Only Connect as series.
I am seeing 100% fail rate for Mobile website bookings.
Does anybody know if there is a protocol for reporting it as a bug? It seems like an irritating little flaw in an essentially excellent system.