Hmm, very odd. I thought maybe it was the router, but that rebooted (as it often does) before my very eyes and no change. I removed the IP address reservation and rebooted. No change. Once again I removed all cables and the sd card and phaffed around for a while and hey presto! New hostname has asserted. I'm now tempted but wary of saying the power needs to be interrupted for a duration of some minutes to discharge all capacitance.
“In the modern age, to call a man unelectable means he cannot be bought”