So … on the suggestion of Sharon Birch (aka @EffieMerryl on Twitter) I tweeted Alan Davies (@alandavies1) who passed it on to David Renwick. And half an hour later Alan Davies reported back as follows:
David said (via email) that if he’d scripted a coincidence like that in the show no-one would have believed it!
and I can see his point. Sometimes life is a lot stranger than you can possibly imagine. I’m now wondering precisely how many bizarre murder weapons there are left to discover, and if this is in fact numerically more than zero. And I’m actually quite glad that I wasn’t able to watch the actual episode last night, because I might quite possibly have had some kind of seizure.
But all credit to Alan and David for taking the time to get back to me.