Following up:
The spare parts arrived yesterday and today I replaced just the hotend thermistor. That seems to have resolved the problem as I did about 6 hours of printing with no events. Yipee!
I remain unclear how a faulty hotend thermistor could have had consequential effects on the reported bed temperature as shown in the chart (from the original problem) so any celebrations might be premature, but no such problem today.
Chart 1 - hotend.png